[Threading] Don't use TLS keys as template arguments.

There's no guarantee that e.g. pthread_key_t is an integral type.  It could
be some kind of struct, or some other thing that isn't valid as a template
argument.

rdar://90776105

include/swift/Threading/TLSKeys.h

@@ -0,0 +1,29 @@
+//===--- TLSKeys.h - Reserved TLS keys ------------------------ -*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2022 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_THREADING_TLSKEYS_H
+#define SWIFT_THREADING_TLSKEYS_H
+
+namespace swift {
+
+enum class tls_key {
+  runtime,
+  stdlib,
+  compatibility50,
+  concurrency_task,
+  concurrency_executor_tracking_info,
+  concurrency_fallback
+};
+
+} // namespace swift
+
+#endif // SWIFT_THREADING_TLSKEYS_H