IR: Specialize MDScope::getFile() for MDFile

Fix `MDScope::getFile()` so that it correctly returns a valid `MDFile`
even when it's an instance of `MDFile`.  This logic is necessary because
of r230057.  I'm working on moving the new hierarchy into place
out-of-tree (on track to commit Monday morning, BTW), and this was
exposed by a few failing tests.

llvm-svn: 230871
2 files changed