fortran: array descriptor: Move absent descriptor initialization [PR122521]

Absent assumed shape arguments are represented by array descriptors whose
data pointer field is initialized to null.  Move the code generating that
initialization to its own function in trans-descriptor.cc.

	PR fortran/122521

gcc/fortran/ChangeLog:

	* trans-expr.cc (gfc_conv_derived_to_class): Move absent array
	descriptor initialization ...
	* trans-descriptor.cc (gfc_init_absent_descriptor): ... here as a
	new function.
	* trans-descriptor.h (gfc_init_absent_descriptor): New declaration.