[flang][openmp] Make Reduction clause part of OmpClause

After discussion in D93105 we found that the reduction clause was not following
the common OmpClause convention. This patch makes reduction clause part of OmpClause
with a value of OmpReductionClause in a similar way than task_reduction.
The unparse function for OmpReductionClause is adapted since the keyword and parenthesis
are issued by the corresponding unparse function for parser::OmpClause::Reduction.

Reviewed By: sameeranjoshi

Differential Revision: https://reviews.llvm.org/D93482