[tsan] Add interceptor for libcxx __shared_count::__release_shared()

We already have an interceptor for __shared_weak_count::__release_shared, this patch handles __shared_count::__release_shared in the same way. This should get rid of TSan false positives when using std::future.

Differential Revision: https://reviews.llvm.org/D27797

llvm-svn: 289831