OpenMP: Add C support for metadirectives and dynamic selectors.

Additional shared C/C++ testcases are included in a subsequent patch in this
series.

gcc/c-family/ChangeLog
	PR middle-end/112779
	PR middle-end/113904
	* c-common.h (enum c_omp_directive_kind): Add C_OMP_DIR_META.
	(c_omp_expand_variant_construct): Declare.
	* c-gimplify.cc: Include omp-general.h.
	(genericize_omp_metadirective_stmt): New.
	(c_genericize_control_stmt): Add case for OMP_METADIRECTIVE.
	* c-omp.cc (c_omp_directives): Fix entries for metadirective.
	(c_omp_expand_variant_construct_r): New.
	(c_omp_expand_variant_construct): New.
	* c-pragma.cc (omp_pragmas): Add metadirective.
	* c-pragma.h (enum pragma_kind): Add PRAGMA_OMP_METADIRECTIVE.

gcc/c/ChangeLog
	PR middle-end/112779
	PR middle-end/113904
	* c-parser.cc (struct c_parser): Add omp_metadirective_state field.
	(c_parser_skip_to_end_of_block_or_statement): Add metadirective_p
	parameter and handle skipping over the parentheses in a "for"
	statement.
	(struct omp_metadirective_parse_data): New.
	(mangle_metadirective_region_label): New.
	(c_parser_label): Mangle label names in a metadirective body.
	(c_parser_statement_after_labels): Likewise.
	(c_parser_pragma): Handle PRAGMA_OMP_METADIRECTIVE.
	(c_parser_omp_context_selector): Allow arbitrary expressions in
	device_num and condition properties.
	(c_parser_omp_assumption_clauses): Handle C_OMP_DIR_META.
	(analyze_metadirective_body): New.
	(c_parser_omp_metadirective): New.

gcc/testsuite/
	PR middle-end/112779
	* c-c++-common/gomp/declare-variant-2.c: Adjust expected output for C.
	* gcc.dg/gomp/metadirective-1.c: New.

Co-Authored-By: Kwok Cheung Yeung <kcy@codesourcery.com>
Co-Authored-By: Sandra Loosemore <sandra@codesourcery.com>