[flang][Lower] Convert OMP Map and related functions to evaluate::Expr

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.