[flang] Fix bug in .mod file for some subprogram attributes

Some attributes for subprograms can be in the subprogram prefix but
others cannot. For the latter, emit a separate attribute statement
to specify them. We were already doing that for PRIVATE but not for
OPTIONAL. Those may be the only two attributes this can apply to.

Fixes flang-compiler/f18#659.

Original-commit: flang-compiler/f18@ae67e087800c5aecb8cb2d2aea16d19b8cc8ccf2
Reviewed-on: https://github.com/flang-compiler/f18/pull/675
Tree-same-pre-rewrite: false