C++, OpenMP: Handle ARRAY_TYPE array sections, not just pointers

This patch is a bug fix for commit gcc-17-202-g3f8c7483112, 'OpenMP:
Expand "declare mapper" mappers for target {enter,exit,} data
directives'.  The ChangeLog for that patch indicated that
cxx_omp_map_array_section was supposed to accept both arrays and
pointers as the base expression, but the code actually only allowed
pointers.  OTOH, both the error-checking in handle_omp_array_sections_1
and the code generation in build_array_ref allow ARRAY_TYPE, and code with
such expressions is actually coming in to the function.

There is no test case included with this patch, but I ran across this
in the two new test cases included with the OpenMP iterators patch
https://gcc.gnu.org/pipermail/gcc-patches/2025-July/689071.html which
I am preparing to resubmit.

gcc/cp/ChangeLog
	* semantics.cc (cxx_omp_map_array_section): Allow ARRAY_TYPE
	expressions, not just pointers.