[OpenACC 'exit data'] Strip 'GOMP_MAP_STRUCT' mappings

[OpenACC 'exit data'] Strip 'GOMP_MAP_STRUCT' mappings

These are not itself necessary for OpenACC 'exit data' directives, and are
skipped over (now) in libgomp.  We might as well not emit them to start with,
in line with the equivalent OpenMP directive.  We keep the no-op handling in
libgomp for the reason of backward compatibility.

	gcc/
	* gimplify.c (gimplify_adjust_omp_clauses): Remove
	'GOMP_MAP_STRUCT' mapping from OpenACC 'exit data' directives.
	gcc/testsuite/
	* c-c++-common/goacc/struct-enter-exit-data-1.c: New file.
	libgomp/
	* oacc-mem.c (goacc_exit_data_internal) <GOMP_MAP_STRUCT>: Explain
	special handling.

Co-Authored-By: Julian Brown <julian@codesourcery.com>