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.

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.

…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.

The related functions are `gatherDataOperandAddrAndBounds` and
`genBoundsOps`. The former is used in OpenACC as well, and it was
updated to pass evaluate::Expr instead of parser objects.

The difference in the test case comes from unfolded conversions
of index expressions, which are explicitly of type integer(kind=8).

Delete now unused `findRepeatableClause2` and `findClause2`.

Add `AsGenericExpr` that takes std::optional. It already returns optional
Expr. Making it accept an optional Expr as input would reduce the number
of necessary checks when handling frequent optional values in evaluator.