This is as straightforward as it sounds, a renamed from shared_mutex to
shared_timed_mutex.
Note that libcxx .dylib and .so files built with c++14 support need to
be rebuilt.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@204078 91177308-0d34-0410-b5e6-96231b3b80d8