[libcxx] Provide thread annotations for shared_mutex
[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: 756ef344af921d95d562d9e9f9389127a89a6314@346567 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx