Work around a linux libc bug causing a crash in TaskPool

Summary:
Doing a pthread_detach while the thread is exiting can cause crashes or other mischief, so we
make sure the thread stays around long enough. The performance impact of the added
synchronization should be minimal, as the parent thread is already holding a mutex, so I am just
making sure it holds it for a little while longer. It's possible the new thread will block on
this mutex immediately after startup, but it should be unblocked really quickly and some
blocking is unavoidable if we actually want to have this synchronization.

Reviewers: tberghammer

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D19153

llvm-svn: 266423