LLDB: Fixed two race conditions when stopping private state thread

When stopping the private state thread, there was a race condition between the time the thread exits (resetting the HostThread object) and the time a Join was attempted, especially in the case of a timeout.

The previous workaround of copying the HostThread object is not enough, since on a Reset the internal thread stuff gets nulled out regardless of which HostThread object actually has Reset called on it, resulting in an attempt to dereference a null pointer on the subsequent call to Join from the copy as well.

Additionally, there was a race between the detach (called when stopping the process) and the stop itself, causing the stop to time out because it was waiting for the private state thread to see the stop state, but it had exited immediately after entering the detached state.

Patch by cameron314

Differential Revision: http://reviews.llvm.org/D19122

llvm-svn: 266733