[Ada] Restore general case for folding comparison of static strings

[Ada] Restore general case for folding comparison of static strings

gcc/ada/

	* exp_ch4.adb (Rewrite_Comparison): Add assertion to confirm
	that evaluation folds comparisons with static operands; when
	folding comparison with non-static operands, the resulting
	literal is non-static.
	* sem_eval.adb (Eval_Relational_Op): Refactor nested IF
	statement for the special case in the THEN branch; move code for
	the "general case" out of the ELSE branch.
	* sem_res.adb (Resolve_Comparison_Op): Only apply a dubious
	special-case for GNATprove in the GNATprove_Mode.