Fortran: Fix ICE on associate of pointer [PR118789]

Fix ICE when associating a pointer to void (c_ptr) by looking at the
compatibility of the type hierarchy.

	PR fortran/118789

gcc/fortran/ChangeLog:

	* trans-stmt.cc (trans_associate_var): Compare pointed to types when
	expr to associate is already a pointer.

gcc/testsuite/ChangeLog:

	* gfortran.dg/associate_73.f90: New test.