[Support] Base RWMutex on std::shared_timed_mutex (C++14)

[Support] Base RWMutex on std::shared_timed_mutex (C++14)

This should have the same semantics. We use std::shared_mutex instead on
MSVC and C++17, std::shared_timed_mutex is less efficient than our
custom implementation on Windows, std::shared_mutex should be faster.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369018 91177308-0d34-0410-b5e6-96231b3b80d8