We're not mapping source locations over correctly yet, so the file name
where an error occurs gets shoved into the diagnostic text, but that's
fine for now.
This also silences the "module 'Blah' not found" error coming from the
importer whenever Swift itself can be responsible for importing the module.
Still to do: if we can't build a Clang module, we shouldn't report that
error and then say it can't be found.
<rdar://problem/14509389>
Swift SVN r18230
