OpenMP: Add OMP_STRUCTURED_BLOCK and GIMPLE_OMP_STRUCTURED_BLOCK.

OpenMP: Add OMP_STRUCTURED_BLOCK and GIMPLE_OMP_STRUCTURED_BLOCK.

In order to detect invalid jumps in and out of intervening code in
imperfectly-nested loops, the front ends need to insert some sort of
marker to identify the structured block sequences that they push into
the inner body of the loop.  The error checking happens in the
diagnose_omp_blocks pass, between gimplification and OMP lowering, so
we need both GENERIC and GIMPLE representations of these markers.
They are removed in OMP lowering so no subsequent passes need to know
about them.

This patch doesn't include any front-end changes to generate the new
data structures.

gcc/cp/ChangeLog
	* constexpr.cc (cxx_eval_constant_expression): Handle
	OMP_STRUCTURED_BLOCK.
	* pt.cc (tsubst_expr): Likewise.

gcc/ChangeLog
	* doc/generic.texi (OpenMP): Document OMP_STRUCTURED_BLOCK.
	* doc/gimple.texi (GIMPLE instruction set): Add
	GIMPLE_OMP_STRUCTURED_BLOCK.
	(GIMPLE_OMP_STRUCTURED_BLOCK): New subsection.
	* gimple-low.cc (lower_stmt): Error on GIMPLE_OMP_STRUCTURED_BLOCK.
	* gimple-pretty-print.cc (dump_gimple_omp_block): Handle
	GIMPLE_OMP_STRUCTURED_BLOCK.
	(pp_gimple_stmt_1): Likewise.
	* gimple-walk.cc (walk_gimple_stmt): Likewise.
	* gimple.cc (gimple_build_omp_structured_block): New.
	* gimple.def (GIMPLE_OMP_STRUCTURED_BLOCK): New.
	* gimple.h (gimple_build_omp_structured_block): Declare.
	(gimple_has_substatements): Handle GIMPLE_OMP_STRUCTURED_BLOCK.
	(CASE_GIMPLE_OMP): Likewise.
	* gimplify.cc (is_gimple_stmt): Handle OMP_STRUCTURED_BLOCK.
	(gimplify_expr): Likewise.
	* omp-expand.cc (GIMPLE_OMP_STRUCTURED_BLOCK): Error on
	GIMPLE_OMP_STRUCTURED_BLOCK.
	* omp-low.cc (scan_omp_1_stmt): Handle GIMPLE_OMP_STRUCTURED_BLOCK.
	(lower_omp_1): Likewise.
	(diagnose_sb_1): Likewise.
	(diagnose_sb_2): Likewise.
	* tree-inline.cc (remap_gimple_stmt): Handle
	GIMPLE_OMP_STRUCTURED_BLOCK.
	(estimate_num_insns): Likewise.
	* tree-nested.cc (convert_nonlocal_reference_stmt): Likewise.
	(convert_local_reference_stmt): Likewise.
	(convert_gimple_call): Likewise.
	* tree-pretty-print.cc (dump_generic_node): Handle
	OMP_STRUCTURED_BLOCK.
	* tree.def (OMP_STRUCTURED_BLOCK): New.
	* tree.h (OMP_STRUCTURED_BLOCK_BODY): New.