[flang][OpenMP] Convert repeatable clauses (except Map) in ClauseProc… (#81623)

…essor

Rename `findRepeatableClause` to `findRepeatableClause2`, and make the
new `findRepeatableClause` operate on new `omp::Clause` objects.

Leave `Map` unchanged, because it will require more changes for it to
work.

[Clause representation 3/6]