[Threading] Create new threading library and use it.

Moved all the threading code to one place.  Added explicit support for
Darwin, Linux, Pthreads, C11 threads and Win32 threads, including new
implementations of Once for Linux, Pthreads, C11 and Win32.

rdar://90776105

include/swift/Threading/Impl/Nothreads.h

@@ -0,0 +1,76 @@
+//==--- Nothreads.h - Threading abstraction implementation ----- -*-C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 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
+//
+//===----------------------------------------------------------------------===//
+//
+// Implements threading support for platforms without threading
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_THREADING_IMPL_NOTHREADS_H
+#define SWIFT_THREADING_IMPL_NOTHREADS_H
+
+namespace swift {
+namespace threading_impl {
+
+// .. Thread related things ..................................................
+
+using thread_id = unsigned;
+
+inline thread_id thread_get_current() { return 0; }
+inline thread_id thread_get_main() { return 0; }
+inline bool thread_is_main() { return true; }
+inline bool threads_same(thread_id a, thread_id b) { return a == b; }
+
+// .. Mutex support ..........................................................
+
+using mutex_handle = unsigned;
+
+inline void mutex_init(mutex_handle &handle, bool checked=false) {}
+inline void mutex_destroy(mutex_handle &handle) { }
+inline void mutex_lock(mutex_handle &handle) { }
+inline void mutex_unlock(mutex_handle &handle) { }
+inline bool mutex_try_lock(mutex_handle &handle) { return true; }
+
+inline void mutex_unsafe_lock(mutex_handle &handle) { }
+inline void mutex_unsafe_unlock(mutex_handle &handle) { }
+
+using lazy_mutex_handle = unsigned;
+
+inline constexpr lazy_mutex_handle lazy_mutex_initializer() { return 0; }
+inline void lazy_mutex_destroy(lazy_mutex_handle &handle) { }
+inline void lazy_mutex_lock(lazy_mutex_handle &handle) { }
+inline void lazy_mutex_unlock(lazy_mutex_handle &handle) { }
+inline bool lazy_mutex_try_lock(lazy_mutex_handle &handle) { return true; }
+
+inline void lazy_mutex_unsafe_lock(lazy_mutex_handle &handle) { }
+inline void lazy_mutex_unsafe_unlock(lazy_mutex_handle &handle) { }
+
+// .. Once ...................................................................
+
+typedef bool once_t;
+
+inline void once(once_t &predicate, void (*fn)(void *), void *ctx) {
+  if (!predicate) {
+    predicate = true;
+    fn(ctx);
+  }
+}
+
+// .. Thread local storage ...................................................
+
+// If we have no threads, we can use the simple version of TLS
+#define SWIFT_THREAD_LOCAL
+
+} // namespace threading_impl
+
+} // namespace swift
+
+#endif // SWIFT_THREADING_IMPL_NOTHREADS_H