[flang] Some initial clean-up in messages, use std::variant to make component...

[flang] Some initial clean-up in messages, use std::variant to make component possibilities clearer.

Original-commit: flang-compiler/f18@9b7eeb23548b75138f3956ec06dbb582da1aa1d9
Reviewed-on: https://github.com/flang-compiler/f18/pull/83
Tree-same-pre-rewrite: false