[flang] Implement new F2023 interoperable enum bind(c) types

The Fortran 2023 Standard updates the enumerator and enumerations. This update is to allow for interoperable enumerations and enum types. This issue is to cover section 7.6.1 of the F2023 standard.

I've heard that there may be ongoing discussion about how C/C++ will be handling enums going forward. This issue should await complete implementation until the corresponding C decisions have been made so that we can maintain interoperability.