Fix deadlock while attaching to inferiors

Summary:
There was a race condition in the AsyncThread, where we would end up sending a vAttach
notification to the thread before it got a chance set up its listener (this can be reproduced by
adding a sleep() at the very beginning of ProcessGDBRemote::AsyncThread()). This event would then
get lost and we LLDB would deadlock. I fix this by setting up the listener early on, in the
ProcessGDBRemote constructor.

This should improve the stability of all attach tests. For now, I am removing XTIMEOUT from
TestAttachResume, and will watch the buildbots for signs of trouble.

Reviewers: clayborg, ovyalov

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D12552

llvm-svn: 246756