[tsan] Avoid calling Block_copy in the "sync" GCD interceptors

The synchronous dispatch functions in GCD (dispatch_sync, dispatch_barrier_sync), don't make a copy of the passed block. To maintain binary compatibility, we should avoid doing that as well in TSan, as there's no reason to do that. The synchronous dispatch functions will not return before the block is actually executed.

rdar://problem/42242579

Differential Revision: https://reviews.llvm.org/D50920

llvm-svn: 340342