[flang][OpenMP] Convert unique clauses in ClauseProcessor (#81622)

Temporarily rename old clause list to `clauses2`, old clause iterator to
`ClauseIterator2`.
Change `findUniqueClause` to iterate over `omp::Clause` objects, modify
all handlers to operate on 'omp::clause::xyz` equivalents.

[Clause representation 2/6]

Author: kparzysz