[flang] Correct handling of USE-associated generic override in nested scope

As the new test here shows by failing with the current compiler with
a bogus error message about indistinguishable specific procedures in
a generic interface, name resolution needs to take care to not
copy a USE-associated generic into the current scope for extension
when the USE association is actually into an enclosing scope.

Differential Revision: https://reviews.llvm.org/D145750