openmp: Add support for OpenMP 5.1 structured-block-sequences

openmp: Add support for OpenMP 5.1 structured-block-sequences

Related to this is the addition of structured-block-sequence in OpenMP 5.1,
which doesn't change anything for Fortran, but for C/C++ allows multiple
statements instead of just one possibly compound around the separating
directives (section and scan).

I've also made some updates to the OpenMP 5.1 support list in libgomp.texi.

2021-10-09  Jakub Jelinek  <jakub@redhat.com>

gcc/c/
	* c-parser.c (c_parser_omp_structured_block_sequence): New function.
	(c_parser_omp_scan_loop_body): Use it.
	(c_parser_omp_sections_scope): Likewise.
gcc/cp/
	* parser.c (cp_parser_omp_structured_block): Remove disallow_omp_attrs
	argument.
	(cp_parser_omp_structured_block_sequence): New function.
	(cp_parser_omp_scan_loop_body): Use it.
	(cp_parser_omp_sections_scope): Likewise.
gcc/testsuite/
	* c-c++-common/gomp/sections1.c (foo): Don't expect errors on
	multiple statements in between section directive(s).  Add testcases
	for invalid no statements in between section directive(s).
	* gcc.dg/gomp/sections-2.c (foo): Don't expect errors on
	multiple statements in between section directive(s).
	* g++.dg/gomp/sections-2.C (foo): Likewise.
	* g++.dg/gomp/attrs-6.C (foo): Add testcases for multiple
	statements in between section directive(s).
	(bar): Add testcases for multiple statements in between scan
	directive.
	* g++.dg/gomp/attrs-7.C (bar): Adjust expected error recovery.
libgomp/
	* libgomp.texi (OpenMP 5.1): Mention implemented support for
	structured block sequences in C/C++.  Mention support for
	unconstrained/reproducible modifiers on order clause.
	Mention partial (C/C++ only) support of extentensions to atomics
	construct.  Mention partial (C/C++ on clause only) support of
	align/allocator modifiers on allocate clause.