OpenMP/Fortran: Ensure allocator is gimplified for 'omp allocate'

OpenMP/Fortran: Ensure allocator is gimplified for 'omp allocate'

Without this change, we we get an ICE in verify_gimple_call for
GOMP_allocate when doing a late replacement in omp-low.cc

gcc/fortran/ChangeLog:

       * trans-openmp.cc (gfc_trans_omp_clauses): Avoid gfc_evaluate_now
       for allocator with indirect ref for better diagnostic.

gcc/ChangeLog:

	* gcc/gimplify.cc (gimplify_omp_allocate): Gimplify allocator.
	* omp-low.cc (lower_omp_allocate): Simplify; GOMP_free can also
	take a plain 0 as allocator argument (arg is unused in libgomp).

libgomp/ChangeLog:

	* testsuite/libgomp.fortran/allocate-8a.f90: New test.