ada: Simplify expansion of packed array assignments

When expanding assignment to a packed array object, e.g. a formal
parameter with mode OUT that might have unconstrained type, we took the
component type and component size from the constrained actual subtype.
It is simpler to take these properties from the nominal type of the
assigned object.

Semantics is unaffected, because constraining the array doesn't change
the type or size of the array components.

gcc/ada/

	* exp_pakd.adb (Expand_Bit_Packed_Element_Set): Change Ctyp and Csiz
	from variables to constants and compute them using the nominal type
	of the assigned array object.