[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
Reviewed By: clementval, kiranktp
Differential Revision: https://reviews.llvm.org/D93641