Replace "Char_Code (UI_To_Int (...))" with "UI_To_CC (...).

Cleanup related to handling characters in GNATprove counterexamples;
semantics is unaffected.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* par-prag.adb (Prag): Simplify conversion of character codes.
	* sem_case.adb (Choice_Image): Likewise.
	(Lit_Of): Likewise.