This patch changes the representation of OMP array sections in the
C++ front end to use the new OMP_ARRAY_SECTION tree code instead of a
TREE_LIST.  This is important for "declare mapper" support, because the
array section representation may stick around longer (in "declare
mapper" definitions), and special-case handling TREE_LIST becomes
necessary in more places, which starts to become unwieldy.

This patch has been rebased, but is largely unchanged from the last
version posted.

Tested alongside previous patch with offloading to NVPTX. OK?

Thanks,

Julian

2022-11-01  Julian Brown  <julian@codesourcery.com>

gcc/c-family/
        * c-omp.cc (c_omp_split_clauses): Support OMP_ARRAY_SECTION.

gcc/cp/
        * parser.cc (cp_parser_omp_var_list_no_open): Use OMP_ARRAY_SECTION
        code instead of TREE_LIST to represent OpenMP array sections.
        * pt.cc (tsubst_copy, tsubst_omp_clause_decl, tsubst_copy_and_build):
        Add OMP_ARRAY_SECTION support.
        * semantics.cc (handle_omp_array_sections_1, handle_omp_array_sections,
        cp_oacc_check_attachments, finish_omp_clauses): Use OMP_ARRAY_SECTION
        instead of TREE_LIST where appropriate.
        * gimplify.cc (gimplify_expr): Ensure OMP_ARRAY_SECTION has been
        processed out before gimplification.