[Ada] Incorrect unreferenced warnings on null subprograms and formals with aspects

[Ada] Incorrect unreferenced warnings on null subprograms and formals with aspects

This patch corrects an error in the compiler whereby spurious
unreferenced warnings are generated on formal parameters of null generic
subprograms.

These changes also fix the parsing of aspects on formal parameters such
that the aspects now get properly set for all formal parameters in a
parameter list.

gcc/ada/

	* par-ch6.adb (P_Formal_Part): Set Aspect_Specifications on all
	formals instead of just the last in a formal id list.
	* sem_ch6.adb (Analyze_Null_Procedure): Mark expanded null
	generic procedures as trivial in order to avoid spurious
	unreferenced warnings.