[flang][OpenMP] Implement flexible OpenMP clause representation

The new set of classes representing OpenMP classes mimics the contents of parser::OmpClause, but differs in a few aspects: - it can be easily created, copied, etc. - is based on semantics::SomeExpr instead of parser objects. The class `OmpObject` is represented by `omp::Object`, which contains the symbol associated with the object, and semantics::MaybeExpr representing the designator for the symbol reference. This patch only introduces the new classes, they are not yet used anywhere.

Author: kparzysz