[flang] Small code cleanups

Use brace-initialization more consistently.
Use std::is_same_v instead of std::is_same and "if constexpr".

Original-commit: flang-compiler/f18@7f461faf9a8df16822a054ec4683f4387c6292e1
Reviewed-on: https://github.com/flang-compiler/f18/pull/173
Tree-same-pre-rewrite: false