Fix the check in StopInfoBreakpoint for "are we currently running an expression"

We were checking "WasTheLastResumeForUserExpression" but that returns true even
if that expression was completed, provided we haven't run again.  This uses a
better check.

This is actually fairly hard to trigger.  It happens the first time you hit an
objc_exception_throw breakpoint and invoke that frame recognizer for that.  But
I couldn't trigger it using a Python based frame recognizer.  So I wrote a test
for the objc_exception_throw_breakpoint recognizer which should have been there
anyway...  It fails (the target auto-continues) w/o this patch and succeeds with
it.

Differential Revision: https://reviews.llvm.org/D147587