Clean up the stop printing header lines.

I added a "thread-stop-format" to distinguish between the form
that is just the thread info (since the stop printing immediately prints
the frame info) and one with more frame 0 info - which is useful for
"thread list" and the like.

I also added a frame.no-debug boolean to the format entities so you can
print frame information differently between frames with source info and those
without.

This closes https://reviews.llvm.org/D26383.
<rdar://problem/28273697>

llvm-svn: 286288