This patch reimplements the omp_target_reorder_clauses function in
anticipation of supporting "deeper" struct mappings (that is, with
several structure dereference operators, or similar).

The idea is that in place of the (possibly quadratic) algorithm in
omp_target_reorder_clauses that greedily moves clauses containing
addresses that are subexpressions of other addresses before those other
addresses, we employ a topological sort algorithm to calculate a proper
order for map clauses. This should run in linear time, and hopefully
handles degenerate cases where multiple "levels" of indirect accesses
are present on a given directive.

The new method also takes care to keep clause groups together, addressing
the concerns raised in:

  https://gcc.gnu.org/pipermail/gcc-patches/2021-May/570501.html

To figure out if some given clause depends on a base pointer in another
clause, we strip off the outer layers of the address expression, and check
(via a tree_operand_hash hash table we have built) if the result is a
"base pointer" as defined in OpenMP 5.0 (1.2.6 Data Terminology). There
are some subtleties involved, however:

 - We must treat MEM_REF with zero offset the same as INDIRECT_REF.
   This should probably be fixed in the front ends instead so we always
   use a canonical form (probably INDIRECT_REF). The following patch
   shows one instance of the problem, but there may be others:

   https://gcc.gnu.org/pipermail/gcc-patches/2021-May/571382.html

 - Mapping a whole struct implies mapping each of that struct's
   elements, which may be base pointers. Because those base pointers
   aren't necessarily explicitly referenced in the directive in question,
   we treat the whole-struct mapping as a dependency instead.

2022-09-13  Julian Brown  <julian@codesourcery.com>

gcc/
	* gimplify.cc (is_or_contains_p, omp_target_reorder_clauses): Delete
	functions.
	(omp_tsort_mark): Add enum.
	(omp_mapping_group): Add struct.
	(debug_mapping_group, omp_get_base_pointer, omp_get_attachment,
	omp_group_last, omp_gather_mapping_groups, omp_group_base,
	omp_index_mapping_groups, omp_containing_struct,
	omp_tsort_mapping_groups_1, omp_tsort_mapping_groups,
	omp_segregate_mapping_groups, omp_reorder_mapping_groups): New
	functions.
	(gimplify_scan_omp_clauses): Call above functions instead of
	omp_target_reorder_clauses, unless we've seen an error.
	* omp-low.cc (scan_sharing_clauses): Avoid strict test if we haven't
	sorted mapping groups.

gcc/testsuite/
	* g++.dg/gomp/target-lambda-1.C: Adjust expected output.
	* g++.dg/gomp/target-this-3.C: Likewise.
	* g++.dg/gomp/target-this-4.C: Likewise.