Use gfc_reset_vptr more consistently.

Use gfc_reset_vptr more consistently.

The vptr for a class type is set in various ways in different
locations.  Refactor the use and simplify code.

gcc/fortran/ChangeLog:

	* trans-array.cc (structure_alloc_comps): Use reset_vptr.
	* trans-decl.cc (gfc_trans_deferred_vars): Same.
	(gfc_generate_function_code): Same.
	* trans-expr.cc (gfc_reset_vptr): Allow supplying the class
	type.
	(gfc_conv_procedure_call): Use reset_vptr.
	* trans-intrinsic.cc (gfc_conv_intrinsic_transfer): Same.