[flang] Allow module procedure and generic with same name

The `GenericDetails::CheckSpecific()` check was happening too early.
We have to wait until all procedures of the generic have been seen.
The generic can have the same name as a module procedure only if that
module procedure is a specific procedure of the generic.

Improve the `SayAlreadyDeclared` error message when the previous
declaration is a use-association

Original-commit: flang-compiler/f18@269e3db6020a5acc51a291dfea98455e6a97d2d5
Reviewed-on: https://github.com/flang-compiler/f18/pull/305
Tree-same-pre-rewrite: false