OpenMP: C front end 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.

New common C/C++ testcases are in a separate patch.

gcc/c-family/ChangeLog
	* c-common.h (c_omp_check_loop_binding_exprs): Declare.
	* c-omp.cc: Include tree-iterator.h.
	(find_binding_in_body): New.
	(check_loop_binding_expr_r): New.
	(LOCATION_OR): New.
	(check_looop_binding_expr): New.
	(c_omp_check_loop_binding_exprs): New.

gcc/c/ChangeLog
	* c-parser.cc (struct c_parser): Add omp_for_parse_state field.
	(struct omp_for_parse_data): New.
	(check_omp_intervening_code): New.
	(add_structured_block_stmt): New.
	(c_parser_compound_statement_nostart): Recognize intervening code,
	nested loops, and other things that need special handling in
	OpenMP loop constructs.
	(c_parser_while_statement): Error on loop in intervening code.
	(c_parser_do_statement): Likewise.
	(c_parser_for_statement): Likewise.
	(c_parser_postfix_expression_after_primary): Error on calls to
	the OpenMP runtime in intervening code.
	(c_parser_pragma): Error on OpenMP pragmas in intervening code.
	(c_parser_omp_loop_nest): New.
	(c_parser_omp_for_loop): Rewrite to use recursive descent, calling
	c_parser_omp_loop_nest to do the heavy lifting.

gcc/ChangeLog
	* omp-api.h: New.
	* omp-general.cc (omp_runtime_api_procname): New.
	(omp_runtime_api_call): Moved here from omp-low.cc, and make
	non-static.
	* omp-general.h: Include omp-api.h.
	* omp-low.cc (omp_runtime_api_call): Delete this copy.

gcc/testsuite/ChangeLog
	* c-c++-common/goacc/collapse-1.c: Update for new C error behavior.
	* c-c++-common/goacc/tile-2.c: Likewise.
	* gcc.dg/gomp/collapse-1.c: Likewise.

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