[flang] Further refine errors vs warnings for ambiguous generics (#80161)

Ensure that the compiler emits a hard error for a generic interface with
ambiguous specific procedures when it is declared as such, and the
ambiguity doesn't involve optional or unlimited polymorphic dummy data
arguments. But: emit an optional portability warning when the ambiguity
in the generic interface is due to USE association's merging of multiple
generics, as USE association may involve modules not under control of
the programmer; we'll emit a hard error message if any the actual
arguments in a particular reference to the generic procedure doesn't
resolve to exactly one specific procedure. And don't emit warnings when
potential ambiguity due to USE association is taking place in a module
file; the warnings, if any, will have been produced when the module file
was compiled.