[Ada] Fix Next_Actual when used on calls "inlined for proof"

[Ada] Fix Next_Actual when used on calls "inlined for proof"

The GNATprove backend needs to apply antialiasing checks to subprogram
calls that have been rewritten into null statements while "inlining for
proof". This requires the First_Actual/Next_Actual to use the Original_Node
and not the N_Null_Statement that rewriting leaves as a parent.

Only effective in GNATprove mode, so no frontend test provided.

2018-07-17  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* sem_util.adb (Next_Actual): If the parent is a N_Null_Statement,
	which happens for inlined calls, then fetch the next actual from the
	original AST.

From-SVN: r262773