Merging r368867 and r368916:

------------------------------------------------------------------------
r368867 | marshall | 2019-08-14 18:21:27 +0200 (Wed, 14 Aug 2019) | 1 line

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
------------------------------------------------------------------------

------------------------------------------------------------------------
r368916 | marshall | 2019-08-14 22:54:56 +0200 (Wed, 14 Aug 2019) | 1 line

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

llvm-svn: 369369