[flang] Merge pull request flang-compiler/f18#237 from flang-compiler/pmk1

Use `std::optional<>::value()` where appropriate in place of `operator*`.

Original-commit: flang-compiler/f18@611ee9be5d610a6593072672b358f1ab038681ad
Reviewed-on: https://github.com/flang-compiler/f18/pull/237