[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

llvm-svn: 346567
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: f30c0117c890fcb8265c556c0a02ab89542473a2
1 file changed