[Ada] Add support for Unreferenced aspect on formal parameters

[Ada] Add support for Unreferenced aspect on formal parameters

gcc/ada/

	* aspects.adb (Has_Aspect_Specifications_Flag): Add
	N_Parameter_Specification.
	* par-ch13.adb (Aspect_Specifications_Present): Also handle case
	of an unknown aspect on the last formal parameter (terminated by
	a Tok_Right_Paren). Minor reformatting.
	* par-ch6.adb (P_Formal_Part): Scan aspects on formal
	parameters.
	* par.adb: Fix typos.
	* sem_ch6.adb (Process_Formals): Add processing of aspects and
	in particular Unreferenced aspect for now.
	* sinfo.ads: Allow ASPECT_SPECIFICATIONS on a
	PARAMETER_SPECIFICATION.
	* doc/gnat_rm/implementation_defined_aspects.rst
	(Aspect Unreferenced): Update documentation.
	* gnat_rm.texi: Regenerate.