ada: Decouple compile-time evaluation from while loop source locations

ada: Decouple compile-time evaluation from while loop source locations

The compile-time evaluation relied on source locations to decide whether a
variable reference occurs within a WHILE loop where the evaluation can
assume the loop condition. Now this relies exclusively on the AST structure.

gcc/ada/ChangeLog:

	* exp_util.adb (Find_In_Enclosing_Context): Refactor from handling of
	IF statements.
	(Get_Current_Value_Condition): Reuse IF code for WHILE statements.