[Foundation] Update & simplify class name stability check

Move the ObjC class name stability check logic to the Swift runtime, exposing it as a new SPI called _swift_isObjCTypeNameSerializable.

Update the reporting logic. The ObjC names of generic classes are considered stable now, but private classes and classes defined in function bodies or other anonymous contexts are unstable by design.

On the overlay side, rewrite the check’s implementation in Swift and considerably simplify it.

rdar://57809977

Author: lorentey

include/swift/Runtime/FoundationSupport.h

@@ -0,0 +1,40 @@
+//===--- FoundationSupport.cpp - Support functions for Foundation ---------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+// Helper functions for the Foundation framework.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_RUNTIME_FOUNDATION_SUPPORT_H
+#define SWIFT_RUNTIME_FOUNDATION_SUPPORT_H
+
+#include "swift/Runtime/Config.h"
+
+#if SWIFT_OBJC_INTEROP
+#include <objc/runtime.h>
+
+#ifdef __cplusplus
+namespace swift { extern "C" {
+#endif
+
+/// Returns a boolean indicating whether the Objective-C name of a class type is
+/// stable across executions, i.e., if the class name is safe to serialize. (The
+/// names of private and local types are unstable.)
+SWIFT_RUNTIME_STDLIB_SPI
+bool _swift_isObjCTypeNameSerializable(Class theClass);
+
+#ifdef __cplusplus
+}} // extern "C", namespace swift
+#endif
+
+#endif // SWIFT_OBJC_INTEROP
+#endif // SWIFT_RUNTIME_FOUNDATION_SUPPORT_H

stdlib/public/Darwin/Foundation/CMakeLists.txt

@@ -7,7 +7,7 @@ add_swift_target_library(swiftFoundation ${SWIFT_SDK_OVERLAY_LIBRARY_BUILD_TYPES
   BundleLookup.mm
   Calendar.swift
   CharacterSet.swift
-  CheckClass.mm
+  CheckClass.swift
   Codable.swift
   Collections+DataProtocol.swift
   CombineTypealiases.swift