Now that the structure of this is more reasonably laid out, fix a long

standing deficiency: we were providing no macro backtrace information
whenever caret diagnostics were turned off. This sinks the logic for
suppressing the code snippet and caret to the code that actually prints
tho code snippet and caret. Along the way, clean up the naming of
functions, remove some now fixed FIXMEs, and generally improve the
wording and logic of this process.

Add a test case exerecising this functionality. It is notable that the
resulting messages are extremely low quality. I'm working on a follow-up
patch that should address this and have left a FIXME in the test case.

llvm-svn: 142120