fortran: array descriptor: Move null actual creation [PR122521]

The creation of a descriptor corresponding to a null() actual argument is
repeated three times in conv_null_actual to handle three different cases.
Factor that code to a common function in to trans-descriptor.cc.

The factored code consisted of calling the scalar descriptor generator,
and then generating a modification of the descriptor rank.  This was
producing correct initialization code for the descriptor, but with a
wrong type and an incorrect zero GFC_TYPE_ARRAY_RANK attached to it.  So
this change additionnally inlines the scalar descriptor generation,
replacing the hardcoded zero rank with the true rank passed as argument, so
that the variable has the right type and is directly generated with the
correct rank.  With this, the descriptor is bigger because of a non-empty
array of useless (and uninitialized) dimensions, but it's the type that is
expected from within the procedure.

More, the scalar descriptor generation, as an argument passing
implementation detail, was generating code copying back to the original
variable on return.  This is not applicable to null() actual arguments and
this change removes it from the inlined code.  That part actually used to
cause a gimplification failure for copy back code that looked like this:
    &C.5048 = (void * *) desc.14.data;
Harald provided a workaround for this with his r15-6408-gd637e6d069ade7 [1]
null() work, that changed the intent attribute of an argument in one of the
three places modified here.  This change, having removed the offending code,
removes the workaround as well.

[1]: https://gcc.gnu.org/pipermail/gcc-patches/2024-December/671673.html

	PR fortran/122521

gcc/fortran/ChangeLog:

	* trans-expr.cc (conv_null_actual): Remove dummy symbol intent
	attribute modification.  Move the three times repeated descriptor
	initialization...
	* trans-descriptor.cc (gfc_create_null_actual_descriptor): ...
	here as a new function and inline the call to
	gfc_conv_scalar_to_descriptor.  Remove the inapplicable or
	redundant parts from the inlined code.  Create lower and upper
	bounds arrays and clear them before passing to the type creation
	function.
	* trans-descriptor.h (gfc_create_null_actual_descriptor): New
	declaration.