OpenMP: C++ support for metadirectives and dynamic selectors.

OpenMP: C++ support for metadirectives and dynamic selectors.

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

gcc/cp/ChangeLog
	PR middle-end/112779
	PR middle-end/113904
	* cp-tree.h (struct saved_scope): Add new field
	x_processing_omp_trait_property_expr.
	(processing_omp_trait_property_expr): New.
	* parser.cc (cp_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.
	(cp_parser_label_for_labeled_statement): Mangle label names in a
	metadirective body.
	(cp_parser_jump_statement): Likewise.
	(cp_parser_omp_context_selector): Allow arbitrary expressions in
	device_num and condition properties.
	(cp_parser_omp_assumption_clauses): Handle C_OMP_DIR_META.
	(analyze_metadirective_body): New.
	(cp_parser_omp_metadirective): New.
	(cp_parser_pragma): Handle PRAGMA_OMP_METADIRECTIVE.
	* parser.h (struct cp_parser): Add omp_metadirective_state field.
	* pt.cc (tsubst_omp_context_selector): New.
	(tsubst_stmt): Handle OMP_METADIRECTIVE.
	* semantics.cc (finish_id_expression_1): Don't diagnose use of
	parameter outside function body in dynamic selector expressions here.

	gcc/testsuite/
	PR middle-end/112779
	PR middle-end/113904
	* c-c++-common/gomp/declare-variant-2.c: Adjust output for C++.
	* g++.dg/gomp/declare-variant-class-1.C: New.
	* g++.dg/gomp/declare-variant-class-2.C: New.
	* g++.dg/gomp/metadirective-template-1.C: New.

	libgomp/
	PR middle-end/112779
	PR middle-end/113904
	* testsuite/libgomp.c++/metadirective-template-1.C: New.
	* testsuite/libgomp.c++/metadirective-template-2.C: New.
	* testsuite/libgomp.c++/metadirective-template-3.C: New.

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