Fix the handling of the run lock in cases where you needed to run

a hand-called function from the private state thread.  The problem 
was that on the way out of the private state thread, we try to drop
the run lock.  That is appropriate for the main private state thread,
but not the secondary private state thread.  Only the thread that 
spawned them can know whether this is an appropriate thing to do or
not.

<rdar://problem/21375352>

llvm-svn: 240461