[Concurrency][Stdlib] Add SwiftStdlibCurrentOS availability, use it.

If you use SwiftStdlibCurrentOS availability, you will be able to
use new types and functions from within the implementation. This
works by, when appropriate, building with the CurrentOS availability
set to the current deployment target.

rdar://150944675

stdlib/public/core/Duration.swift

@@ -31,7 +31,7 @@
 /// temporal measurement component; specifically leap seconds should be 
 /// represented as an additional accessor since that is specific only to certain
 /// clock implementations.
-@available(SwiftStdlib 5.7, *)
+@available(SwiftStdlibCurrentOS 5.7, *)
 @frozen
 public struct Duration: Sendable {
   /// The low 64 bits of a 128-bit signed integer value counting attoseconds.
@@ -86,13 +86,13 @@ public struct Duration: Sendable {
   }
 }
 
-@available(SwiftStdlib 5.7, *)
+@available(SwiftStdlibCurrentOS 5.7, *)
 extension Duration {
   /// The composite components of the `Duration`.
   ///
   /// This is intended for facilitating conversions to existing time types. The
   /// attoseconds value will not exceed 1e18 or be lower than -1e18.
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public var components: (seconds: Int64, attoseconds: Int64) {
     let (seconds, attoseconds) = _attoseconds.dividedBy1e18()
     return (Int64(seconds), Int64(attoseconds))
@@ -129,7 +129,7 @@ extension Duration {
   }
 }
 
-@available(SwiftStdlib 5.7, *)
+@available(SwiftStdlibCurrentOS 5.7, *)
 extension Duration {
   /// Construct a `Duration` given a number of seconds represented as a 
   /// `BinaryInteger`.
@@ -137,7 +137,7 @@ extension Duration {
   ///       let d: Duration = .seconds(77)
   ///
   /// - Returns: A `Duration` representing a given number of seconds.
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   @inlinable
   public static func seconds<T: BinaryInteger>(_ seconds: T) -> Duration {
     guard let high = Int64(exactly: seconds >> 64) else { fatalError() }
@@ -170,7 +170,7 @@ extension Duration {
   ///       let d: Duration = .seconds(22.93)
   ///
   /// - Returns: A `Duration` representing a given number of seconds.
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public static func seconds(_ seconds: Double) -> Duration {
     Duration(seconds, scale: 1_000_000_000_000_000_000)
   }
@@ -181,7 +181,7 @@ extension Duration {
   ///       let d: Duration = .milliseconds(645)
   ///
   /// - Returns: A `Duration` representing a given number of milliseconds.
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   @inlinable
   public static func milliseconds<T: BinaryInteger>(
     _ milliseconds: T
@@ -199,7 +199,7 @@ extension Duration {
   ///       let d: Duration = .milliseconds(88.3)
   ///
   /// - Returns: A `Duration` representing a given number of milliseconds.
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public static func milliseconds(_ milliseconds: Double) -> Duration {
     Duration(milliseconds, scale: 1_000_000_000_000_000)
   }
@@ -210,7 +210,7 @@ extension Duration {
   ///       let d: Duration = .microseconds(12)
   ///
   /// - Returns: A `Duration` representing a given number of microseconds.
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   @inlinable
   public static func microseconds<T: BinaryInteger>(
     _ microseconds: T
@@ -228,7 +228,7 @@ extension Duration {
   ///       let d: Duration = .microseconds(382.9)
   ///
   /// - Returns: A `Duration` representing a given number of microseconds.
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public static func microseconds(_ microseconds: Double) -> Duration {
     Duration(microseconds, scale: 1_000_000_000_000)
   }
@@ -239,7 +239,7 @@ extension Duration {
   ///       let d: Duration = .nanoseconds(1929)
   ///
   /// - Returns: A `Duration` representing a given number of nanoseconds.
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   @inlinable
   public static func nanoseconds<T: BinaryInteger>(
     _ nanoseconds: T
@@ -263,10 +263,10 @@ extension Duration {
   }
 }
 
-@available(SwiftStdlib 5.7, *)
+@available(SwiftStdlibCurrentOS 5.7, *)
 @_unavailableInEmbedded
 extension Duration: Codable {
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public init(from decoder: Decoder) throws {
     var container = try decoder.unkeyedContainer()
     let high = try container.decode(Int64.self)
@@ -274,7 +274,7 @@ extension Duration: Codable {
     self.init(_high: high, low: low)
   }
 
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public func encode(to encoder: Encoder) throws {
     var container = encoder.unkeyedContainer()
     try container.encode(_high)
@@ -282,112 +282,112 @@ extension Duration: Codable {
   }
 }
 
-@available(SwiftStdlib 5.7, *)
+@available(SwiftStdlibCurrentOS 5.7, *)
 extension Duration: Hashable {
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public func hash(into hasher: inout Hasher) {
     hasher.combine(_attoseconds)
   }
 }
 
-@available(SwiftStdlib 5.7, *)
+@available(SwiftStdlibCurrentOS 5.7, *)
 extension Duration: Equatable {
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public static func == (_ lhs: Duration, _ rhs: Duration) -> Bool {
     return lhs._attoseconds == rhs._attoseconds
   }
 }
 
-@available(SwiftStdlib 5.7, *)
+@available(SwiftStdlibCurrentOS 5.7, *)
 extension Duration: Comparable {
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public static func < (_ lhs: Duration, _ rhs: Duration) -> Bool {
     return lhs._attoseconds < rhs._attoseconds
   }
 }
 
-@available(SwiftStdlib 5.7, *)
+@available(SwiftStdlibCurrentOS 5.7, *)
 extension Duration: AdditiveArithmetic {
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public static var zero: Duration { Duration(_attoseconds: 0) }
 
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public static func + (_ lhs: Duration, _ rhs: Duration) -> Duration {
     return Duration(_attoseconds: lhs._attoseconds + rhs._attoseconds)
   }
 
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public static func - (_ lhs: Duration, _ rhs: Duration) -> Duration {
     return Duration(_attoseconds: lhs._attoseconds - rhs._attoseconds)
   }
 
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public static func += (_ lhs: inout Duration, _ rhs: Duration) {
     lhs = lhs + rhs
   }
 
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public static func -= (_ lhs: inout Duration, _ rhs: Duration) {
     lhs = lhs - rhs
   }
 }
 
-@available(SwiftStdlib 5.7, *)
+@available(SwiftStdlibCurrentOS 5.7, *)
 extension Duration {
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public static func / (_ lhs: Duration, _ rhs: Double) -> Duration {
     return Duration(_attoseconds:
       _Int128(Double(lhs._attoseconds) / rhs))
   }
 
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public static func /= (_ lhs: inout Duration, _ rhs: Double) {
     lhs = lhs / rhs
   }
 
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public static func / <T: BinaryInteger>(
     _ lhs: Duration, _ rhs: T
   ) -> Duration {
     Duration(_attoseconds: lhs._attoseconds / _Int128(rhs))
   }
 
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public static func /= <T: BinaryInteger>(_ lhs: inout Duration, _ rhs: T) {
     lhs = lhs / rhs
   }
 
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public static func / (_ lhs: Duration, _ rhs: Duration) -> Double {
     Double(lhs._attoseconds) / Double(rhs._attoseconds)
   }
 
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public static func * (_ lhs: Duration, _ rhs: Double) -> Duration {
     Duration(_attoseconds: _Int128(Double(lhs._attoseconds) * rhs))
   }
 
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public static func * <T: BinaryInteger>(
     _ lhs: Duration, _ rhs: T
   ) -> Duration {
     Duration(_attoseconds: lhs._attoseconds * _Int128(rhs))
   }
 
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public static func *= <T: BinaryInteger>(_ lhs: inout Duration, _ rhs: T) {
     lhs = lhs * rhs
   }
 }
 
-@available(SwiftStdlib 5.7, *)
+@available(SwiftStdlibCurrentOS 5.7, *)
 @_unavailableInEmbedded
 extension Duration: CustomStringConvertible {
-  @available(SwiftStdlib 5.7, *)
+  @available(SwiftStdlibCurrentOS 5.7, *)
   public var description: String {
     return (Double(_attoseconds) / 1e18).description + " seconds"
   }
 }
 
-@available(SwiftStdlib 5.7, *)
+@available(SwiftStdlibCurrentOS 5.7, *)
 extension Duration: DurationProtocol { }