Arrow operator handling for C front-end in OpenMP map clauses

Arrow operator handling for C front-end in OpenMP map clauses

This patch merges some of the equivalent changes already done for the C++
front-end to the C parts.

2021-03-08  Chung-Lin Tang  <cltang@codesourcery.com>

gcc/c/ChangeLog:

	* c-parser.c (c_parser_omp_clause_map): Set 'allow_deref' argument in
	call to c_parser_omp_variable_list to 'true'.
	* c-typeck.c (handle_omp_array_sections_1): Add strip of MEM_REF in
	array base handling.
	(c_finish_omp_clauses): Handle 'A->member' case in map clauses.

gcc/ChangeLog:

	* gimplify.c (gimplify_scan_omp_clauses): Add MEM_REF case when
	handling component_ref_p case. Add unshare_expr and gimplification
	when created GOMP_MAP_STRUCT is not a DECL. Add code to add
	firstprivate pointer for *pointer-to-struct case.

gcc/testsuite/ChangeLog:

	* gcc.dg/gomp/target-3.c: New test.