Expose __SwiftNativeNSString for Foundation's use, as well as an initializer to create Strings from them

Author: Catfish-Man

stdlib/public/core/GroupInfo.json

@@ -33,6 +33,7 @@
     "StringRangeReplaceableCollection.swift",
     "StringGutsRangeReplaceable.swift",
     "StringStorage.swift",
+    "StringStorageBridge.swift",
     "StringSwitch.swift",
     "StringTesting.swift",
     "StringUTF16View.swift",

stdlib/public/core/StringBridge.swift

@@ -658,7 +658,7 @@ internal func _SwiftCreateBridgedString_DoNotCall(
 // This allows us to subclass an Objective-C class and use the fast Swift
 // memory allocator.
 @objc @_swift_native_objc_runtime_base(__SwiftNativeNSStringBase)
-class __SwiftNativeNSString {
+@_spi(Foundation) public class __SwiftNativeNSString {
   @objc internal init() {}
   deinit {}
 }

stdlib/public/core/StringStorageBridge.swift

@@ -17,6 +17,28 @@ import SwiftShims
 internal let _cocoaASCIIEncoding:UInt = 1 /* NSASCIIStringEncoding */
 internal let _cocoaUTF8Encoding:UInt = 4 /* NSUTF8StringEncoding */
 
+extension String {
+  @available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
+  @_spi(Foundation)
+  public init?(_nativeStorage: AnyObject) {
+    let knownOther = _KnownCocoaString(_nativeStorage)
+    switch knownOther {
+    case .storage:
+      self = _unsafeUncheckedDowncast(
+        _nativeStorage,
+        to: __StringStorage.self
+      ).asString
+    case .shared:
+      self = _unsafeUncheckedDowncast(
+        _nativeStorage,
+        to: __SharedStringStorage.self
+      ).asString
+    default:
+      return nil
+    }
+  }
+}
+
 // ObjC interfaces.
 extension _AbstractStringStorage {
   @inline(__always)