ada: Reuse and simplify counting the length of various lists

Use utility routines to compute number of elements in lists of nodes and chains
of formal parameters. Code cleanup; semantics is unaffected.

gcc/ada/ChangeLog:

	* exp_aggr.adb (Others_Check): Reuse List_Length; tune whitespace.
	* exp_ch3.adb (Constrain_Array): Remove counting of constraints.
	* exp_disp.adb (Make_DT): Use Elist utility routines; tune whitespace.
	* sem_ch12.adb (Formal_Dimensions): Reuse List_Length.
	(Valid_Default_Attribute): Reuse Number_Formals.
	* sem_prag.adb (Process_Convention): Likewise.
	* sem_util.adb (Normalize_Actuals): Likewise.
	(Check_Function_Writable_Actuals): Reuse List_Length.