[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