[Support] Re-introduce the RWMutexImpl for macOS < 10.12
In r369018, Benjamin replaced the custom RWMutex implementation with
their C++14 counterpart. Unfortunately, std::shared_timed_mutex is only
available on macOS 10.12 and later. This prevents LLVM from compiling
even on newer versions of the OS when you have an older deployment
target. This patch reintroduced the old RWMutexImpl but guards it by the
macOS availability macro.
Differential revision: https://reviews.llvm.org/D66313