Rather than deferring to the heavyweight validateDeclForNameLookup()
to perform the “get overridden decls” operation, perform a much more
minimal validation that only looks for exact matches when the ‘override’
modifier is present.
The more-extensive checking (e.g., that one didn’t forget an override
modifier) is only performed as part of the “full” type checking of
a declaration, which will typically only be done within the same source
file as the declaration. The intent here is to reduce the amount of
work performed to check overrides cross-file, and limit the dependencies
of the “get overridden decls” operation.
