OpenMP: C++ support for imperfectly-nested loops

OpenMP 5.0 removed the restriction that multiple collapsed loops must
be perfectly nested, allowing "intervening code" (including nested
BLOCKs) before or after each nested loop.  In GCC this code is moved
into the inner loop body by the respective front ends.

This patch changes the C++ front end to use recursive descent parsing
on nested loops within an "omp for" construct, rather than an
iterative approach, in order to preserve proper nesting of compound
statements.  Preserving cleanups (destructors) for class objects
declared in intervening code and loop initializers complicates moving
the former into the body of the loop; this is handled by parsing the
entire construct before reassembling any of it.

gcc/cp/ChangeLog
	* cp-tree.h (cp_convert_omp_range_for): Adjust declaration.
	* parser.cc (struct omp_for_parse_data): New.
	(cp_parser_postfix_expression): Diagnose calls to OpenMP runtime
	in intervening code.
	(check_omp_intervening_code): New.
	(cp_parser_statement_seq_opt): Special-case nested loops, blocks,
	and other constructs for OpenMP loops.
	(cp_parser_iteration_statement): Reject loops in intervening code.
	(cp_parser_omp_for_loop_init): Expand comments and tweak the
	interface slightly to better distinguish input/output parameters.
	(cp_convert_omp_range_for): Likewise.
	(cp_parser_omp_loop_nest): New, split from cp_parser_omp_for_loop
	and largely rewritten.  Add more comments.
	(insert_structured_blocks): New.
	(find_structured_blocks): New.
	(struct sit_data, substitute_in_tree_walker, substitute_in_tree):
	New.
	(fixup_blocks_walker): New.
	(cp_parser_omp_for_loop): Rewrite to use recursive descent instead
	of a loop.  Add logic to reshuffle the bits of code collected
	during parsing so intervening code gets moved to the loop body.
	(cp_parser_omp_loop): Remove call to finish_omp_for_block, which
	is now redundant.
	(cp_parser_omp_simd): Likewise.
	(cp_parser_omp_for): Likewise.
	(cp_parser_omp_distribute): Likewise.
	(cp_parser_oacc_loop): Likewise.
	(cp_parser_omp_taskloop): Likewise.
	(cp_parser_pragma): Reject OpenMP pragmas in intervening code.
	* parser.h (struct cp_parser): Add omp_for_parse_state field.
	* pt.cc (tsubst_omp_for_iterator): Adjust call to
	cp_convert_omp_range_for.
	* semantics.cc (finish_omp_for): Try harder to preserve location
	of loop variable init expression for use in diagnostics.
	(struct fofb_data, finish_omp_for_block_walker): New.
	(finish_omp_for_block): Allow variables to be bound in a BIND_EXPR
	nested inside BIND instead of directly in BIND itself.

gcc/testsuite/ChangeLog
	* c-c++-common/goacc/tile-2.c: Adjust expected error patterns.
	* g++.dg/gomp/attrs-imperfect1.C: New test.
	* g++.dg/gomp/attrs-imperfect2.C: New test.
	* g++.dg/gomp/attrs-imperfect3.C: New test.
	* g++.dg/gomp/attrs-imperfect4.C: New test.
	* g++.dg/gomp/attrs-imperfect5.C: New test.
	* g++.dg/gomp/pr41967.C: Adjust expected error patterns.
	* g++.dg/gomp/tpl-imperfect-gotos.C: New test.
	* g++.dg/gomp/tpl-imperfect-invalid-scope.C: New test.

libgomp/ChangeLog
	* testsuite/libgomp.c++/attrs-imperfect1.C: New test.
	* testsuite/libgomp.c++/attrs-imperfect2.C: New test.
	* testsuite/libgomp.c++/attrs-imperfect3.C: New test.
	* testsuite/libgomp.c++/attrs-imperfect4.C: New test.
	* testsuite/libgomp.c++/attrs-imperfect5.C: New test.
	* testsuite/libgomp.c++/attrs-imperfect6.C: New test.
	* testsuite/libgomp.c++/imperfect-class-1.C: New test.
	* testsuite/libgomp.c++/imperfect-class-2.C: New test.
	* testsuite/libgomp.c++/imperfect-class-3.C: New test.
	* testsuite/libgomp.c++/imperfect-destructor.C: New test.
	* testsuite/libgomp.c++/imperfect-template-1.C: New test.
	* testsuite/libgomp.c++/imperfect-template-2.C: New test.
	* testsuite/libgomp.c++/imperfect-template-3.C: New test.