[Threading] Add ConditionVariable support.

Swift Concurrency would like to be able to use condition variables.
Add support to the threading packages.

rdar://100236038

include/swift/Threading/ScopedLock.h

@@ -0,0 +1,58 @@
+//===--- ScopedLock.h - ScopedLock ---------------------------- -*- 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
+//
+//===----------------------------------------------------------------------===//
+//
+// Provides a ScopedLockT utility template that is used by Mutex and
+// ConditionVariable.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_THREADING_SCOPEDLOCK_H
+#define SWIFT_THREADING_SCOPEDLOCK_H
+
+namespace swift {
+
+// -- ScopedLock ---------------------------------------------------------------
+
+/// Compile time adjusted stack based object that locks/unlocks the supplied
+/// Mutex type. Use the provided typedefs instead of this directly.
+template <typename T, bool Inverted>
+class ScopedLockT {
+  ScopedLockT() = delete;
+  ScopedLockT(const ScopedLockT &) = delete;
+  ScopedLockT &operator=(const ScopedLockT &) = delete;
+  ScopedLockT(ScopedLockT &&) = delete;
+  ScopedLockT &operator=(ScopedLockT &&) = delete;
+
+public:
+  explicit ScopedLockT(T &l) : Lock(l) {
+    if (Inverted) {
+      Lock.unlock();
+    } else {
+      Lock.lock();
+    }
+  }
+
+  ~ScopedLockT() {
+    if (Inverted) {
+      Lock.lock();
+    } else {
+      Lock.unlock();
+    }
+  }
+
+private:
+  T &Lock;
+};
+
+} // namespace swift
+
+#endif // SWIFT_THREADING_SCOPEDLOCK_H