[Ada] Rewrite Iterate_Call_Parameters in more assertive style

[Ada] Rewrite Iterate_Call_Parameters in more assertive style

The formal and actual parameters in a subprogram call must match each other.
This is now checked with assertion (so that we can detect possible mistakes),
while the production builds have less work to do. Semantics unchanged.

2018-05-25  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* sem_util.adb (Iterate_Call_Parameters): Rewrite with extra
	assertions; replace function versions of Next_Formal/Next_Actual with
	their procedural versions (which are more concise).

From-SVN: r260725