openmp: Add C support for parsing metadirectives

openmp: Add C support for parsing metadirectives

This patch implements parsing for the OpenMP metadirective introduced in
OpenMP 5.0.  Metadirectives are parsed into an OMP_METADIRECTIVE node,
with the variant clauses forming a chain accessible via
OMP_METADIRECTIVE_CLAUSES.  Each clause contains the context selector
and tree for the variant.

User conditions in the selector are now permitted to be non-constant when
used in metadirectives as specified in OpenMP 5.1.

2021-01-25  Kwok Cheung Yeung  <kcy@codesourcery.com>

	gcc/
	* omp-general.c (omp_context_selector_matches): Add extra argument.
	(omp_resolve_metadirective): New stub function.
	* omp-general.h (struct omp_metadirective_variant): New.
	(omp_context_selector_matches): Add extra argument.
	(omp_resolve_metadirective): New prototype.
	* tree.def (OMP_METADIRECTIVE): New.
	* tree.h (OMP_METADIRECTIVE_CLAUSES): New macro.

	gcc/c/
	* c-parser.c (c_parser_skip_to_end_of_block_or_statement): Handle
	parentheses in statement.
	(c_parser_omp_metadirective): New prototype.
	(c_parser_omp_context_selector): Add extra argument.  Allow
	non-constant expressions.
	(c_parser_omp_context_selector_specification): Add extra argument and
	propagate it to c_parser_omp_context_selector.
	(analyze_metadirective_body): New.
	(c_parser_omp_metadirective): New.
	(c_parser_omp_construct): Handle PRAGMA_OMP_METADIRECTIVE.

	gcc/c-family
	* c-common.h (enum c_omp_directive_kind): Add C_OMP_DIR_META.
	(c_omp_expand_metadirective): New prototype.
	* c-gimplify.c (genericize_omp_metadirective_stmt): New.
	(c_genericize_control_stmt): Handle OMP_METADIRECTIVE tree nodes.
	* c-omp.c (omp_directives): Classify metadirectives as C_OMP_DIR_META.
	(c_omp_expand_metadirective): New stub function.
	* c-pragma.c (omp_pragmas): Add entry for metadirective.
	* c-pragma.h (enum pragma_kind): Add PRAGMA_OMP_METADIRECTIVE.