Improve diagnostics range reporting.

Summary:
If we have some range information coming from clang diagnostic, promote
that one even if it doesn't contain diagnostic location inside.

Reviewers: sammccall, ioeric

Reviewed By: ioeric

Subscribers: ilya-biryukov, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D52544

llvm-svn: 343197