AST: Introduce a Swift runtime availability domain.

Add support for the `Swift` availability domain, which represents availability
with respect to the Swift runtime. Use of this domain is restricted by the
experimental feature `SwiftRuntimeAvailability`.

include/swift/AST/AvailabilityDomain.h

@@ -51,6 +51,9 @@ public:
     /// Represents availability with respect to Swift language mode.
     SwiftLanguageMode,
 
+    /// Represents availability with respect to the Swift runtime.
+    SwiftRuntime,
+
     /// Represents PackageDescription availability.
     PackageDescription,
 
@@ -144,6 +147,10 @@ public:
     return AvailabilityDomain(Kind::SwiftLanguageMode);
   }
 
+  static AvailabilityDomain forSwiftRuntime() {
+    return AvailabilityDomain(Kind::SwiftRuntime);
+  }
+
   static AvailabilityDomain forPackageDescription() {
     return AvailabilityDomain(Kind::PackageDescription);
   }
@@ -185,6 +192,8 @@ public:
     return getKind() == Kind::SwiftLanguageMode;
   }
 
+  bool isSwiftRuntime() const { return getKind() == Kind::SwiftRuntime; }
+
   bool isPackageDescription() const {
     return getKind() == Kind::PackageDescription;
   }
@@ -236,6 +245,10 @@ public:
   bool isActivePlatform(const ASTContext &ctx,
                         bool forTargetVariant = false) const;
 
+  /// Returns true if availability in this domain must be specified alone in
+  /// `@available` attributes and `if #available` queries.
+  bool mustBeSpecifiedAlone() const;
+
   /// Returns the domain's minimum available range for type checking. For
   /// example, for the domain of the platform that compilation is targeting,
   /// this version is specified with the `-target` option. For the Swift

include/swift/Basic/LangOptions.h

@@ -188,6 +188,9 @@ namespace swift {
     /// Swift runtime version to compile for.
     version::Version RuntimeVersion = version::Version::getCurrentLanguageVersion();
 
+    /// The minimum Swift runtime version that the progam can be deployed to.
+    version::Version MinSwiftRuntimeVersion;
+
     /// PackageDescription version to compile for.
     version::Version PackageDescriptionVersion;
 

include/swift/Option/Options.td

@@ -298,6 +298,13 @@ def language_mode : Separate<["-"], "language-mode">,
   MetaVarName<"<mode>">,
   Alias<swift_version>;
 
+def min_swift_runtime_version
+    : Separate<["-"], "min-swift-runtime-version">,
+      Flags<[FrontendOption, ModuleInterfaceOptionIgnorable]>,
+      HelpText<"The minimum Swift runtime version "
+               "that will be available at runtime">,
+      MetaVarName<"<vers>">;
+
 def package_description_version: Separate<["-"], "package-description-version">,
   Flags<[FrontendOption, HelpHidden, ModuleInterfaceOption]>,
   HelpText<"The version number to be applied on the input for the PackageDescription availability kind">,