Ada: Fix iterator filter ignored on enumeration type with clause

The problem comes from an oversight in Expand_N_Loop_Statement.

gcc/ada/
	PR ada/121316
	* exp_ch5.adb (Expand_Iterator_Loop_Over_Array): Minor tweak.
	(Expand_Iterator_Loop_Over_Container): Likewise.
	(Expand_N_Loop_Statement): Use Statements (N) throughout.

gcc/testsuite/
	* gnat.dg/iter7.adb: New test.

Co-authored-by: Liam Powell <liam@liampwll.com>