Implement variadic lock_guard.

Summary:
This patch implements the variadic `lock_guard` paper. 

Making `lock_guard` variadic is a ABI breaking change because the specialization `lock_guard<_Mutex>` mangles differently then when it was the primary template. This change only provides variadic `lock_guard` in ABI V2 or when `_LIBCPP_ABI_VARIADIC_LOCK_GUARD` is defined.

Note that in ABI V2 `lock_guard` must always be declared as a variadic template, even in C++03, in order to keep the ABI consistent. For this reason `lock_guard` is forward declared as a variadic template in all standard dialects and therefore depends on variadic templates being provided as an extension in C++03. All supported versions of Clang and GCC provide this extension.




Reviewers: mclow.lists

Subscribers: K-ballo, mclow.lists, cfe-commits

Differential Revision: http://reviews.llvm.org/D21260

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@272634 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/thread/thread.mutex/thread.lock/thread.lock.guard/variadic_mutex_mangling.pass.cpp

@@ -0,0 +1,31 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// UNSUPPORTED: libcpp-has-no-threads
+
+// THIS TESTS C++03 EXTENSIONS.
+
+// <mutex>
+
+// template <class ...Mutex> class lock_guard;
+
+// Test that the the variadic lock guard implementation mangles the same in
+// C++11 and C++03. This is important since the mangling of `lock_guard` depends
+// on it being declared as a variadic template, even in C++03.
+
+#define _LIBCPP_ABI_VARIADIC_LOCK_GUARD
+#include <mutex>
+#include <typeinfo>
+#include <cassert>
+#include <iostream>
+
+int main() {
+    const std::string expect = "NSt3__110lock_guardIJNS_5mutexEEEE";
+    assert(typeid(std::lock_guard<std::mutex>).name() == expect);
+}