[stdlib] Add LegacyABI.swift for legacy non-API declarations

Author: milseman

stdlib/public/core/CMakeLists.txt

@@ -90,6 +90,7 @@ set(SWIFTLIB_ESSENTIAL
   KeyValuePairs.swift
   LazyCollection.swift
   LazySequence.swift
+  LegacyABI.swift
   LifetimeManager.swift
   ManagedBuffer.swift
   Map.swift

stdlib/public/core/GroupInfo.json

@@ -221,6 +221,7 @@
     "Equatable.swift",
     "Comparable.swift",
     "Codable.swift",
+    "LegacyABI.swift",
     "MigrationSupport.swift"
   ],
   "Result": [

stdlib/public/core/LegacyABI.swift

@@ -0,0 +1,15 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2019 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
+//
+//===----------------------------------------------------------------------===//
+
+// This file contains non-API (or underscored) declarations that are needed to
+// be kept around for ABI compatibility
+