[flang] Dump function return attributes

Also, change SubprogramDetails::result_ from `std::optional<Symbol *>`
to `Symbol *`. We don't need two levels of optional-ness.

Original-commit: flang-compiler/f18@db3b874946dc9b139009d7e10832edb2a076a7bc
Reviewed-on: https://github.com/flang-compiler/f18/pull/368
Tree-same-pre-rewrite: false