Make sure the thread list is updated before you set the stop reason

Make sure the thread list is updated before you set the stop reason
on a thread.  When talking to some older gdb-remote stubs, We were getting
a stop reason from the stop reply packet and setting it on the relevant
thread before we updated the full stop list.  That would get discarded when
the full list was updated.

Also, if you already have a thread list when you go to see if there is an
Operating System plugin, and you do indeed load a new OS plugin, you have to
re-fetch the thread list or it will only show the raw threads.

Differential Revision: https://reviews.llvm.org/D62887

llvm-svn: 364666