[flang] [OpenMP] Simplify `OmpObject` with `std::variant`

Instead of using an `ENUM_CLASS Kind` to distinguish a `Designator`
or common-block name, change it to `std::variant`. If the `Name`
is available, then it is common-block name.

Original-commit: flang-compiler/f18@abf5db6171bc57eeddb58d22cd09e2344b023cad