[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/Once.h

@@ -0,0 +1,35 @@
+//===--- Once.h - Runtime support for lazy initialization -------*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2017 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
+//
+//===----------------------------------------------------------------------===//
+//
+// Swift runtime functions in support of lazy initialization.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_THREADING_ONCE_H
+#define SWIFT_THREADING_ONCE_H
+
+#include "Impl.h"
+
+namespace swift {
+
+using once_t = threading_impl::once_t;
+
+/// Runs the given function with the given context argument exactly once.
+/// The predicate argument must refer to a global or static variable of static
+/// extent of type swift::once_t.
+inline void once(once_t &predicate, void (*fn)(void *), void *context=nullptr) {
+  threading_impl::once(predicate, fn, context);
+}
+
+}
+
+#endif // SWIFT_THREADING_ONCE_H