[Support] Base SmartMutex on std::recursive_mutex

- Remove support for non-recursive mutexes. This was unused.
- The std::recursive_mutex is now created/destroyed unconditionally.
  Locking is still only done if threading is enabled.
- Alias SmartScopedLock to std::lock_guard.

This should make no semantic difference on the existing APIs.

llvm-svn: 368158