re PR c/53580 (Internal Segmentation fault in nested "omp parallel", "omp parallel for" and "omp parallel for reduction" Directives)

	PR middle-end/53580
	* omp-low.c (scan_omp): Change first argument to
	gimple_seq *, call walk_gimple_seq_mod instead of
	walk_gimple_seq.
	(scan_sharing_clauses, scan_omp_parallel, scan_omp_task,
	scan_omp_for, scan_omp_sections, scan_omp_single,
	execute_lower_omp): Adjust callers.
	(scan_omp_1_stmt): Likewise.  If check_omp_nesting_restrictions
	returns false, replace stmt with GIMPLE_NOP.
	(check_omp_nesting_restrictions): Instead of issuing warnings,
	issue errors and return false if any errors were reported.

	* gcc.dg/gomp/nesting-1.c: Expect errors rather than warnings.
	* gcc.dg/gomp/critical-4.c: Likewise.
	* gfortran.dg/gomp/appendix-a/a.35.1.f90: Likewise.
	* gfortran.dg/gomp/appendix-a/a.35.3.f90: Likewise.
	* gfortran.dg/gomp/appendix-a/a.35.4.f90: Likewise.
	* gfortran.dg/gomp/appendix-a/a.35.6.f90: Likewise.
	* c-c++-common/gomp/pr53580.c: New test.

	* testsuite/libgomp.c/pr26943-2.c: Remove #pragma omp barrier,
	use GOMP_barrier () call instead.
	* testsuite/libgomp.c/pr26943-3.c: Likewise.
	* testsuite/libgomp.c/pr26943-4.c: Likewise.
	* testsuite/libgomp.fortran/vla4.f90: Remove !$omp barrier,
	call GOMP_barrier instead.
	* testsuite/libgomp.fortran/vla5.f90: Likewise.

From-SVN: r188298