More hacking on mapDiagnosticRanges to make it handle more cases.

This still isn't quite right, but it fixes a crash.

I factored out findCommonParent because we need it on the result of 
getImmediateExpansionRange: for a function macro, the beginning
and end of an expansion range can come out of different
macros/macro arguments, which means the resulting range is a complete
mess to handle consistently.

I also made some changes to how findCommonParent works; it works somewhat
better in some cases, and somewhat worse in others, but I think overall
it's a better balance.  I'm coming to the conclusion that mapDiagnosticRanges
isn't using the right algorithm, though: chasing the caret is fundamentally
more complicated than any algorithm which only considers one FileID for the
caret can handle because each SourceLocation doesn't really have a single parent.
We need to follow the same path of choosing expansion locations and spelling
locations which the caret used to come up with the correct range
in the general case.

Fixes <rdar://problem/12847524>.

llvm-svn: 170049