[libcxx] Provide thread annotations for shared_mutex

shared_mutex was introduced in C++17 but its implementation currently
doesn't use Clang's thread annotations like regular mutex. This change
adds those.

Differential Revision: https://reviews.llvm.org/D54290

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@346567 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed