Fix a race in ProcessGDBRemote::MonitorDebugServerProcess

Summary:
MonitorDebugServerProcess went to a lot of effort to make sure its asynchronous invocation does
not cause any mischief, but it was still not race-free. Specifically, in a quick stop-restart
sequence (like the one in TestAddressBreakpoints) the copying of the process shared pointer via
target_sp->GetProcessSP() was racing with the resetting of the pointer in DeleteCurrentProcess,
as they were both accessing the same shared_ptr object.

To avoid this, I simply pass in a weak_ptr to the process when the callback is created. Locking
this pointer is race-free as they are two separate object even though they point to the same
process instance. This also removes the need for the complicated tap-dance around retrieving the
process pointer.

Reviewers: clayborg

Subscribers: tberghammer, lldb-commits

Differential Revision: http://reviews.llvm.org/D20107

llvm-svn: 269281