Fix bug in ProcessGDBRemote

Summary:
ProcessGDBRemote::AsyncThread nuked its own thread handle upon exiting. This prevented the main
thread from joining it correctly in StopAsyncThread. I address this by moving the Reset() call to
StopAsyncThread, after the join.

Reviewers: clayborg, zturner

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D8218

llvm-svn: 231915