openacc: Fix standalone attach for Fortran assumed-shape array pointers

openacc: Fix standalone attach for Fortran assumed-shape array pointers

This patch makes it so that an "attach" operation for a Fortran pointer
with an array descriptor copies that array descriptor to the target,
and similarly that detach operations release the array descriptor.

2020-07-16  Julian Brown  <julian@codesourcery.com>
	    Thomas Schwinge  <thomas@codesourcery.com>

gcc/fortran/
	* trans-openmp.c (gfc_trans_omp_clauses): Rework OpenACC
	attach/detach handling for arrays with descriptors.

gcc/testsuite/
	* gfortran.dg/goacc/attach-descriptor.f90: New test.

libgomp/
	* testsuite/libgomp.oacc-fortran/attach-descriptor-1.f90: New test.
	* testsuite/libgomp.oacc-fortran/attach-descriptor-2.f90: New test.

Co-Authored-By: Thomas Schwinge <thomas@codesourcery.com>