ada: Elide temporary for aliased array with unconstrained nominal subtype

When the array is initialized with the result of a call to a function whose
result type is unconstrained, then the result is allocated with its bounds,
so the array can be rewritten as a renaming of the result in this case too.

gcc/ada/

	* exp_ch3.adb (Expand_N_Object_Declaration): Fold initialization
	expression of Nominal_Subtype_Is_Constrained_Array constant into
	the computation of Rewrite_As_Renaming and remove the constant.
	Set it to True for an aliased array with unconstrained nominal
	subtype if the subtype of the expression is also unconstrained.