openmp: Add C++ support for parsing metadirectives

openmp: Add C++ support for parsing metadirectives

This adds support for parsing OpenMP metadirectives in the C++ front end.

2022-01-25  Kwok Cheung Yeung  <kcy@codesourcery.com>

	gcc/cp/
	* parser.cc (cp_parser_skip_to_end_of_statement): Handle parentheses.
	(cp_parser_skip_to_end_of_block_or_statement): Likewise.
	(cp_parser_omp_context_selector): Add extra argument.  Allow
	non-constant expressions.
	(cp_parser_omp_context_selector_specification): Add extra argument and
	propagate to cp_parser_omp_context_selector.
	(analyze_metadirective_body): New.
	(cp_parser_omp_metadirective): New.
	(cp_parser_omp_construct): Handle PRAGMA_OMP_METADIRECTIVE.
	(cp_parser_pragma): Handle PRAGMA_OMP_METADIRECTIVE.