[asan] Intercept and wrap XPC callback blocks

On recent OS X systems, blocks used as callbacks for XPC events (set up e.g. via xpc_connection_set_event_handler) are not later executed via the public libdispatch API (dispatch_async, etc). Because we don't intercept the path where the block is executed, we can fail to register the newly created dispatch thread. To fix that, let's intercept libxpc's APIs that take a block as a callback handler, and let's wrap these blocks in the same way as we do for libdispatch API.

Differential Revision: http://reviews.llvm.org/D12490

llvm-svn: 246961