[flang] Strengthen checking for unresolved names

Make it a fatal error to have an unresolved name, except for a few
exceptions.

Remove namelist, bind, save statements from the exceptions.

Original-commit: flang-compiler/f18@b16cbed780964e48dcba41548ce92dcf42716a98
Reviewed-on: https://github.com/flang-compiler/f18/pull/281