[bolt] Use llvm::sys::RWMutex instead of std::shared_timed_mutex

This has the following advantages:
- std::shared_timed_mutex is macOS 10.12+ only. llvm::sys::RWMutex
  automatically switches to a different implementation internally
  when targeting older macOS versions.
- bolt only needs std::shared_mutex, not std::shared_timed_mutex.
  llvm::sys::RWMutex automatically uses std::shared_mutex internally
  where available.

std::shared_mutex and RWMutex have the same API, so no code changes
other than types and includes are needed.

Differential Revision: https://reviews.llvm.org/D138423