[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.

Author: kparzysz

flang/include/flang/Evaluate/tools.h

@@ -148,6 +148,14 @@ inline Expr<SomeType> AsGenericExpr(Expr<SomeType> &&x) { return std::move(x); }
 std::optional<Expr<SomeType>> AsGenericExpr(DataRef &&);
 std::optional<Expr<SomeType>> AsGenericExpr(const Symbol &);
 
+// Propagate std::optional from input to output.
+template <typename A>
+std::optional<Expr<SomeType>> AsGenericExpr(std::optional<A> &&x) {
+  if (!x)
+    return std::nullopt;
+  return AsGenericExpr(std::move(*x));
+}
+
 template <typename A>
 common::IfNoLvalue<Expr<SomeKind<ResultType<A>::category>>, A> AsCategoryExpr(
     A &&x) {