[flang] Implement new F2023 enumeration types

The Fortran 2023 Standard has added a new enumeration type. This is separate from the F2018 enum and even the F2023 enum type. It is, to quote the standard:

"An enumeration type is a nonintrinsic type with no type parameter. It is not a derived type and is not interoperable."

This new type will be a substantial change. It is documented in section 7.6.2 of the F2023 standard.

It might be wise to implement this in multiple smaller PRs.