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:🧵🆔:operator==() attempting to call
pthread_equal(3) with zero values.
Reported by: andrew@tao11.riddles.org.uk
PR: 239038, 239550
MFC after: 3 days
23cc093cf6