Fix a bug where if we stop but nobody says there was a reason for the stop, we would return

control to the user anyway.  This was put in to handle monitors that would say there was no
stop reason when you first attached to them.  But it broke the case where you hit a thread specific
breakpoint on many threads, but NOT the one specified in the breakpoint.  I work around this
by only doing the junky override when the StopID is 0 - i.e. on first attach.

This commit also adds a test for thread specific breakpoints.

llvm-svn: 199290