IR: Use getRawScope() when verifying

Fix a couple of places in the Verifier that call `getScope()` instead of
`getRawScope()`.  Both DIDerivedType::getScope and
DICompositeType::getScope return a DITypeRef right now (which wraps a
Metadata*) so I don't think there's currently an observable bug.  I
found this because a future commit that will change them to cast to
DIScope*.

llvm-svn: 266552