openmp: Add omp::decl support for C2X

openmp: Add omp::decl support for C2X

This patch adds omp::decl support which has been added recently for
C++ also to C.

2023-11-04  Jakub Jelinek  <jakub@redhat.com>

	* c-parser.h (c_maybe_parse_omp_decl): Declare.
	* c-parser.cc (struct c_parser): Add in_omp_decl_attribute member.
	(c_parser_std_attribute): Uncoment omp::decl handling.
	(c_parser_omp_var_list_parens): If parser->in_omp_decl_attribute
	don't expect any arguments, instead create clause or TREE_LIST for
	that decl.
	(c_maybe_parse_omp_decl): New function.
	(c_parser_omp_declare_target): If parser->in_omp_decl_attribute and
	first token isn't name or comma invoke c_parser_omp_var_list_parens.
	* c-decl.cc (c_decl_attributes): Uncomment omp::decl handling and
	use *node rather than non-existing *decl.

	* gcc.dg/gomp/attrs-19.c: New test.
	* gcc.dg/gomp/attrs-20.c: New test.
	* gcc.dg/gomp/attrs-21.c: New test.