Expose immortal String initialization for Foundation (#78661)

Author: Catfish-Man

stdlib/public/core/StringBridge.swift

@@ -661,6 +661,19 @@ internal func _SwiftCreateBridgedString_DoNotCall(
   return Unmanaged<AnyObject>.passRetained(str._bridgeToObjectiveCImpl())
 }
 
+@available(SwiftStdlib 6.1, *)
+@_spi(Foundation) public func _SwiftCreateImmortalString_ForFoundation(
+  buffer: UnsafeBufferPointer<UInt8>,
+  isASCII: Bool
+) -> String? {
+  switch validateUTF8(buffer) {
+  case .success(let extraInfo):
+    return String(_StringGuts(buffer, isASCII: extraInfo.isASCII))
+  default:
+    return nil
+  }
+}
+
 // At runtime, this class is derived from `__SwiftNativeNSStringBase`,
 // which is derived from `NSString`.
 //

test/abi/macOS/arm64/stdlib.swift

@@ -827,3 +827,6 @@ Added: _$ss4SlabVsRi__rlE5countSivpZMV
 
 // Slab._storage _read accessor
 Added: _$ss4SlabVsRi__rlE8_storagexq_BVvr
+
+// Shared string creation for Foundation
+Added: _$ss40_SwiftCreateImmortalString_ForFoundation6buffer7isASCIISSSgSRys5UInt8VG_SbtF

test/abi/macOS/x86_64/stdlib.swift

@@ -828,3 +828,6 @@ Added: _$ss4SlabVsRi__rlE5countSivpZMV
 
 // Slab._storage _read accessor
 Added: _$ss4SlabVsRi__rlE8_storagexq_BVvr
+
+// Shared string creation for Foundation
+Added: _$ss40_SwiftCreateImmortalString_ForFoundation6buffer7isASCIISSSgSRys5UInt8VG_SbtF