[Ada] Move folding of unchecked conversions from expansion to evaluation

[Ada] Move folding of unchecked conversions from expansion to evaluation

gcc/ada/

	* exp_ch4.adb (Expand_N_Unchecked_Type_Conversion): Remove
	folding of discrete values.
	* exp_intr.adb (Expand_Unc_Conversion): Analyze, resolve and
	evaluate (if possible) calls to instances of
	Ada.Unchecked_Conversion after they have been expanded into
	N_Unchecked_Type_Conversion.
	* sem_eval.adb (Eval_Unchecked_Conversion): Add folding of
	discrete values.