[TSan][libdispatch] Replace CFRunLoop with dispatch_semaphore, pt. 1

Remove the dependency on Foundation so we can start running those tests
on other platforms. Rename/move of tests will be done in a separate
commit.

Reviewed By: kubamracek, dvyukov

Differential Revision: https://reviews.llvm.org/D60347

llvm-svn: 358023