TestExprDoesntBlock started failing because deadlocks behave differently with
pthread_mutex and std::mutex.
This reverts part of commit r245234.
llvm-svn: 245262
This eliminates portability issues among platforms that don't have
a pthreads implementation.
Differential Revision: http://reviews.llvm.org/D12043
Reviewed By: Greg Clayton
llvm-svn: 245234