[flang] Better handling of merged generics

If a generic name is use-associated from two different modules and they
both have a specific procedure or both have a derived type with the same
name, report it as an error.

Original-commit: flang-compiler/f18@42369af96d3cca7ca2050592e728e04de8124bd8
Reviewed-on: https://github.com/flang-compiler/f18/pull/741
Tree-same-pre-rewrite: false