[tsan] Implement basic GCD interceptors for OS X

We need to intercept libdispatch APIs (dispatch_sync, dispatch_async, etc.) to add synchronization between the code that submits the task and the code that gets executed (possibly on a different thread). This patch adds release+acquire semantics for dispatch_sync, and dispatch_async (plus their "_f" and barrier variants). The synchronization is done on malloc'd contexts (separate for each submitted block/callback). Added tests to show usage of dispatch_sync and dispatch_async, for cases where we expect no warnings and for cases where TSan finds races.

Differential Revision: http://reviews.llvm.org/D14745

llvm-svn: 253982