There seems to be some odd corner case where we shut down the...

There seems to be some odd corner case where we shut down the ProcessGDBRemote, but we haven't managed to shut down the async thread.  That causes the ProcessGDBRemote::AsyncThread to crash when it wakes up.  So I changed StartAsyncThread and StopAsyncThread to be callable multiple times (only the first one does anything) so that we can just shut it down unequivocally in the ProcessGDBRemote destructor.

<rdar://problem/12602981>

llvm-svn: 167197