Fortran: Add support for OMP non-rectangular loops.

Fortran: Add support for OMP non-rectangular loops.

This patch adds support for OMP 5.1 "canonical loop nest form" to the
Fortran front end, marks non-rectangular loops for processing
by the middle end, and implements missing checks in the gimplifier
for additional prohibitions on non-rectangular loops.

Note that the OMP spec also prohibits non-rectangular loops with the TILE
construct; that construct hasn't been implemented yet, so that error will
need to be filled in later.

	gcc/fortran/
	* gfortran.h (struct gfc_omp_clauses): Add non_rectangular bit.
	* openmp.cc (is_outer_iteration_variable): New function.
	(expr_is_invariant): New function.
	(bound_expr_is_canonical): New function.
	(resolve_omp_do): Replace existing non-rectangularity error with
	check for canonical form and setting non_rectangular bit.
	* trans-openmp.cc (gfc_trans_omp_do): Transfer non_rectangular
	flag to generated tree structure.

	gcc/
	* gimplify.cc (gimplify_omp_for): Update messages for SCHEDULED
	and ORDERED clause conflict errors.  Add check for GRAINSIZE and
	NUM_TASKS on TASKLOOP.

	gcc/testsuite/
	* c-c++-common/gomp/loop-6.c (f3): New function to test TASKLOOP
	diagnostics.
	* gfortran.dg/gomp/collapse1.f90: Update expected messages.
	* gfortran.dg/gomp/pr85313.f90: Remove dg-error on non-rectangular
	loops that are now accepted.
	* gfortran.dg/gomp/non-rectangular-loop.f90: New file.
	* gfortran.dg/gomp/canonical-loop-1.f90: New file.
	* gfortran.dg/gomp/canonical-loop-2.f90: New file.