[flang][OpenMP] Lower iterator modifiers in map and motion clauses

Add iterator lowering for `target data`, `target enter/exit data`,
`target update`, and map clauses inside `declare mapper`.

For example:

```fortran
!$omp target update to(iterator(i=1:n): a(i), b)
```

Conceptually, this becomes:

```text
entries = omp.iterator(i = 1 through n) {
  yield map_info(base = a, bounds = element i)
}
target_update(iterated_maps = entries, ordinary_maps = [b])
```

Keeping the array base and expressing selections through bounds lets the
same approach handle array elements, sections, and descriptor-backed arrays.

Each locator is expanded only over the iterators that appear in it:

```text
iterator(i=1:n, j=1:m): a(i), b(j), c(i,j)
                        |     |      |
                        i     j    i x j
```

Iterator ranges use the same canonical loop semantics throughout lowering:

```text
trip_count = canonicalTripCount(lower, upper, step, inclusive)

if any trip_count == 0:
    result = []
    skip iterator region
```

These rules preserve source behavior in edge cases. Fortran ranges include
the endpoint, so lowering marks them inclusive instead of dropping the last
entry. A range such as `5:1:1` is empty and yields no entries instead of being
mistaken for a large range. Bounds and reconstructed iterator values keep
their source integer width, preventing `INTEGER(16)` values from being
truncated.

Supports explicit and default mappers, including mapper declarations
imported from other modules.

This PR stacked on top of #197047 and #197752.

This patch is part of feature work for #188061.

Assisted with codex.