[Ada] Raise Constraint_Error when converting negative values to Char_Code

[Ada] Raise Constraint_Error when converting negative values to Char_Code

GNATprove relies on the comment for Get_Enum_Lit_From_Pos, which
promises to raise Constraint_Error when its Pos parameter is not among
the representation values for enumeration literal. However, this promise
was only respected in builds with range checks enabled.

The root problem was that a similar comment for conversion from Uint to
Char_Code was likewise only respected in builds with range checks
enabled.

Now both routines respect promises in their comments. The behaviour of
GNAT itself is not affected. The fix is needed to filter garbage
counterexamples generated by provers for characters objects in SPARK.

gcc/ada/

	* uintp.adb (UI_To_CC): Guard against illegal inputs; reuse
	UI_To_Int.