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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230871 91177308-0d34-0410-b5e6-96231b3b80d8