[Flang][openmp][2/5] Make Default clause part of OmpClause

After discussion in `D93482` we found that the some of the clauses were not
following the common OmpClause convention.

The benefits of using OmpClause:
- Functionalities from structure checker are mostly aligned to work with
  `llvm::omp::Clause`.
- The unparsing as well can take advantage.
- Homogeneity with OpenACC and rest of the clauses in OpenMP.
- Could even generate the parser with TableGen, when there is homogeneity.
- It becomes confusing when to use `flangClass` and `flangClassValue` inside
  TableGen, if incase we generate parser using TableGen we could have only a
  single `let expression`.

This patch makes `OmpDefaultClause` clause part of `OmpClause`.
The unparse function is dropped as the unparsing is done by `WALK_NESTED_ENUM`
for `OmpDefaultClause`.

Reviewed By: clementval, kiranktp

Differential Revision: https://reviews.llvm.org/D93641