Pull in r368867 from upstream libc++ trunk (by Marshall Clow):

Pull in r368867 from upstream libc++ trunk (by Marshall Clow):

  Rework recursive_timed_mutex so that it uses __thread_id instead of
  using the lower-level __libcpp_thread_id. This is prep for fixing
  PR42918. Reviewed as https://reviews.llvm.org/D65895

Pull in r368916 from upstream libc++ trunk (by Marshall Clow):

  Fix thread comparison by making sure we never pass our special 'not a
  thread' value to the underlying implementation. Fixes PR#42918.

This should fix std::thread::id::operator==() attempting to call
pthread_equal(3) with zero values.

Reported by:	andrew@tao11.riddles.org.uk
PR:		239038, 239550
MFC after:	3 days