openmp: Add support for omp attributes section and scan directives

openmp: Add support for omp attributes section and scan directives

This patch adds support for expressing the section and scan directives
using the attribute syntax and additionally fixes some bugs in the attribute
syntax directive handling.
For now it requires that the scan and section directives appear as the only
attribute, not combined with other OpenMP or non-OpenMP attributes on the same
statement.

2021-07-26  Jakub Jelinek  <jakub@redhat.com>

	* parser.h (struct cp_lexer): Add orphan_p member.
	* parser.c (cp_parser_statement): Don't change in_omp_attribute_pragma
	upon restart from CPP_PRAGMA handling.  Fix up condition when a lexer
	should be destroyed and adjust saved_tokens if it records tokens from
	the to be destroyed lexer.
	(cp_parser_omp_section_scan): New function.
	(cp_parser_omp_scan_loop_body): Use it.  If
	parser->lexer->in_omp_attribute_pragma, allow optional comma
	after scan.
	(cp_parser_omp_sections_scope): Use cp_parser_omp_section_scan.

	* g++.dg/gomp/attrs-1.C: Use attribute syntax even for section
	and scan directives.
	* g++.dg/gomp/attrs-2.C: Likewise.
	* g++.dg/gomp/attrs-6.C: New test.
	* g++.dg/gomp/attrs-7.C: New test.
	* g++.dg/gomp/attrs-8.C: New test.