[flang][OpenMP] Implement flexible OpenMP clause representation

Introduce a set of generic classes (templates) that represent OpenMP
clauses in a language-agnostic manner. OpenMP clauses can contain
expressions and data objects and the exact representation of each
depends on the source language of the compiled program. To deal with
this, the templates depend on two type parameters:
- IdType: type that represent object's identity (in a way that
  satisfied OpenMP requirements), and
- ExprType: type that can represent numeric values, as well as
  data references (e.g. x.y[1].z[2]).

In addition to that, implement code instantiating these templates
from flang's AST.

This patch only introduces the new classes, they are not yet used
anywhere.