Fix PR72715 "ICE in gfc_trans_omp_do, at fortran/trans-openmp.c:3164"

Fix PR72715 "ICE in gfc_trans_omp_do, at fortran/trans-openmp.c:3164"

The OpenACC 'resolve_oacc_nested_loops' function duplicates most code of the
OpenMP 'resolve_omp_do', but didn't include the PR60127 "ICE with OpenMP and DO
CONCURRENT" (trunk r210331) changes.  (Probably the two functions should be
unified?)

The Fortran DO CONCURRENT construct is a way to tell the compiler that loop
iterations don't have any interdependencies -- which is information that would
very well be suitable for OpenACC/OpenMP loops.  There are some "details"
however, see the discussion/references in PR60127, so for the time being, make
this a compile-time error instead of an ICE.

	gcc/fortran/
	* openmp.c (resolve_oacc_nested_loops): Error on do concurrent
	loops.

	gcc/testsuite/
	* gfortran.dg/goacc/loop-3-2.f95: Error on do concurrent loops.
	* gfortran.dg/goacc/loop-3.f95: Likewise.
	* gfortran.dg/goacc/pr72715.f90: New test.

Reviewed-by: Thomas Schwinge <thomas@codesourcery.com>
From-SVN: r268875