This commit reworks how the thread plan's ShouldStopHere mechanism works, so...

This commit reworks how the thread plan's ShouldStopHere mechanism works, so that it is useful not only
for customizing "step-in" behavior (e.g. step-in doesn't step into code with no debug info), but also 
the behavior of step-in/step-out and step-over when they step out of the frame they started in.

I also added as a proof of concept of this reworking a mode for stepping where stepping out of a frame
into a frame with no debug information will continue stepping out till it arrives at a frame that does
have debug information.  This is useful when you are debugging callback based code where the callbacks
are separated from the code that initiated them by some library glue you don't care about, among other
things.

llvm-svn: 203747