ada: Implement type inference for generic parameters

...based on previous work that added Gen_Assocs_Rec.
Minor cleanup of that previous work.

gcc/ada/

	* sem_ch12.adb: Implement type inference for generic parameters.
	(Maybe_Infer_One): Forbid inference of anonymous subtypes and
	types.
	(Inference_Reason): Fix comment.
	* debug.adb: Document -gnatd_I switch.
	* errout.ads: Document that Empty is not allowed for "&".
	* errout.adb (Set_Msg_Insertion_Node): Minor: Do not allow
	Error_Msg_Node_1 = Empty for "&". Use "in" instead of multiple
	"=". Improve comment.