[Ada] Fix search for "for ... of" loop subprograms

[Ada] Fix search for "for ... of" loop subprograms

This patch makes the search for Get_Element_Access, Step (Next/Prev),
Reference_Control_Type, and Pseudo_Reference (for optimized "for ... of"
loops) more robust.  In particular, we have a new Next procedure in Ada
2022, and we need to pick the right one.

We have not yet added the new Next and other subprograms.

gcc/ada/

	* exp_ch5.adb (Expand_Iterator_Loop_Over_Container): For each
	subprogram found, assert that the variable is Empty, so we can
	detect bugs where we find two or more things with the same name.
	Without this patch, that bug would happen when we add the new
	Next procedure.  For Step, make sure we pick the right one, by
	checking name and number of parameters.  For Get_Element_Access,
	check that we're picking a function.  That's not really
	necessary, because there is no procedure with that name, but it
	seems cleaner this way.
	* rtsfind.ads: Minor comment improvement. It seems kind of odd
	to say "under no circumstances", and then immediately contradict
	that with "The one exception is...".