[Stdlib][Overlays] Rename various classes to avoid conflicting ObjC names.

Old Swift and new Swift runtimes and overlays need to coexist in the same process. This means there must not be any classes which have the same ObjC runtime name in old and new, because the ObjC runtime doesn't like name collisions.

When possible without breaking source compatibility, classes were renamed in Swift, which results in a different ObjC name.

Public classes were renamed only on the ObjC side using the @_objcRuntimeName attribute.

This is similar to the work done in pull request #19295. That only renamed @objc classes. This renames all of the others, since even pure Swift classes still get an ObjC name.

rdar://problem/46646438

stdlib/public/core/StringStorage.swift

@@ -126,10 +126,10 @@ extension _AbstractStringStorage {
     switch knownOther {
     case .storage:
       return _nativeIsEqual(
-        _unsafeUncheckedDowncast(other, to: _StringStorage.self))
+        _unsafeUncheckedDowncast(other, to: __StringStorage.self))
     case .shared:
       return _nativeIsEqual(
-        _unsafeUncheckedDowncast(other, to: _SharedStringStorage.self))
+        _unsafeUncheckedDowncast(other, to: __SharedStringStorage.self))
 #if !(arch(i386) || arch(arm))
     case .tagged:
       fallthrough
@@ -172,7 +172,10 @@ private typealias CountAndFlags = _StringObject.CountAndFlags
 // Optional<_StringBreadcrumbs>.
 //
 
-final internal class _StringStorage
+// NOTE: older runtimes called this class _StringStorage. The two
+// must coexist without conflicting ObjC class names, so it was
+// renamed. The old name must not be used in the new runtime.
+final internal class __StringStorage
   : __SwiftNativeNSString, _AbstractStringStorage {
 #if arch(i386) || arch(arm)
   // The total allocated storage capacity. Note that this includes the required
@@ -299,7 +302,7 @@ final internal class _StringStorage
 
   @objc(copyWithZone:)
   final internal func copy(with zone: _SwiftNSZone?) -> AnyObject {
-    // While _StringStorage instances aren't immutable in general,
+    // While __StringStorage instances aren't immutable in general,
     // mutations may only occur when instances are uniquely referenced.
     // Therefore, it is safe to return self here; any outstanding Objective-C
     // reference will make the instance non-unique.
@@ -350,13 +353,13 @@ private func determineCodeUnitCapacity(_ desiredCapacity: Int) -> Int {
 }
 
 // Creation
-extension _StringStorage {
+extension __StringStorage {
   @_effects(releasenone)
   private static func create(
     realCodeUnitCapacity: Int, countAndFlags: CountAndFlags
-  ) -> _StringStorage {
+  ) -> __StringStorage {
     let storage = Builtin.allocWithTailElems_2(
-      _StringStorage.self,
+      __StringStorage.self,
       realCodeUnitCapacity._builtinWordValue, UInt8.self,
       1._builtinWordValue, Optional<_StringBreadcrumbs>.self)
 #if arch(i386) || arch(arm)
@@ -380,12 +383,12 @@ extension _StringStorage {
   @_effects(releasenone)
   private static func create(
     capacity: Int, countAndFlags: CountAndFlags
-  ) -> _StringStorage {
+  ) -> __StringStorage {
     _internalInvariant(capacity >= countAndFlags.count)
 
     let realCapacity = determineCodeUnitCapacity(capacity)
     _internalInvariant(realCapacity > capacity)
-    return _StringStorage.create(
+    return __StringStorage.create(
       realCodeUnitCapacity: realCapacity, countAndFlags: countAndFlags)
   }
 
@@ -394,11 +397,11 @@ extension _StringStorage {
     initializingFrom bufPtr: UnsafeBufferPointer<UInt8>,
     capacity: Int,
     isASCII: Bool
-  ) -> _StringStorage {
+  ) -> __StringStorage {
     let countAndFlags = CountAndFlags(
       mortalCount: bufPtr.count, isASCII: isASCII)
     _internalInvariant(capacity >= bufPtr.count)
-    let storage = _StringStorage.create(
+    let storage = __StringStorage.create(
       capacity: capacity, countAndFlags: countAndFlags)
     let addr = bufPtr.baseAddress._unsafelyUnwrappedUnchecked
     storage.mutableStart.initialize(from: addr, count: bufPtr.count)
@@ -409,14 +412,14 @@ extension _StringStorage {
   @_effects(releasenone)
   internal static func create(
     initializingFrom bufPtr: UnsafeBufferPointer<UInt8>, isASCII: Bool
-  ) -> _StringStorage {
-    return _StringStorage.create(
+  ) -> __StringStorage {
+    return __StringStorage.create(
       initializingFrom: bufPtr, capacity: bufPtr.count, isASCII: isASCII)
   }
 }
 
 // Usage
-extension _StringStorage {
+extension __StringStorage {
   @inline(__always)
   private var mutableStart: UnsafeMutablePointer<UInt8> {
     return UnsafeMutablePointer(Builtin.projectTailElems(self, UInt8.self))
@@ -504,7 +507,7 @@ extension _StringStorage {
 }
 
 // Appending
-extension _StringStorage {
+extension __StringStorage {
   // Perform common post-RRC adjustments and invariant enforcement.
   @_effects(releasenone)
   private func _postRRCAdjust(newCount: Int, newIsASCII: Bool) {
@@ -564,7 +567,7 @@ extension _StringStorage {
 }
 
 // Removing
-extension _StringStorage {
+extension __StringStorage {
   @_effects(releasenone)
   internal func remove(from lower: Int, to upper: Int) {
     _internalInvariant(lower <= upper)
@@ -646,7 +649,10 @@ extension _StringStorage {
 }
 
 // For shared storage and bridging literals
-final internal class _SharedStringStorage
+// NOTE: older runtimes called this class _SharedStringStorage. The two
+// must coexist without conflicting ObjC class names, so it was
+// renamed. The old name must not be used in the new runtime.
+final internal class __SharedStringStorage
   : __SwiftNativeNSString, _AbstractStringStorage {
   internal var _owner: AnyObject?
   internal var start: UnsafePointer<UInt8>
@@ -775,7 +781,7 @@ final internal class _SharedStringStorage
 
   @objc(copyWithZone:)
   final internal func copy(with zone: _SwiftNSZone?) -> AnyObject {
-    // While _StringStorage instances aren't immutable in general,
+    // While __StringStorage instances aren't immutable in general,
     // mutations may only occur when instances are uniquely referenced.
     // Therefore, it is safe to return self here; any outstanding Objective-C
     // reference will make the instance non-unique.
@@ -786,7 +792,7 @@ final internal class _SharedStringStorage
 
 }
 
-extension _SharedStringStorage {
+extension __SharedStringStorage {
 #if !INTERNAL_CHECKS_ENABLED
   @inline(__always)
   internal func _invariantCheck() {}