[flang] Fix bug when writing empty generic to .mod file

When a generic interface had no specific procedures, we were writing
it the `.mod` file as `generic::g=>`, which is not valid Fortran.

Change to writing generics as interface blocks rather than generic
statements so that this case is handled. Include an access stmt if it
was declared private.

Also fix a bug in `test_errors.sh` where the expected/actual error
messages weren't sorted by line number correctly.

Original-commit: flang-compiler/f18@1c32a289b53d787c0033ed45033db86afde33458
Reviewed-on: https://github.com/flang-compiler/f18/pull/614
Tree-same-pre-rewrite: false