Add a return value for nasty_mutex::operator&. Patch from STL@microsoft.com

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@267961 91177308-0d34-0410-b5e6-96231b3b80d8

test/support/nasty_containers.hpp

@@ -287,7 +287,7 @@ public:
      nasty_mutex() _NOEXCEPT {}
      ~nasty_mutex() {}
 
-	nasty_mutex *operator& ()   { assert(false); }
+	nasty_mutex *operator& ()   { assert(false); return nullptr; }
 	template <typename T>
 	void operator, (const T &) { assert(false); }