ada: Small cleanup in Expand_N_Object_Declaration

This reuses a local constant more consistently, removes a duplicate of this
local constant, renames local variables, alphabetizes declarations, makes a
few consistency tweaks and adjusts a couple of comments.

No functional changes.

gcc/ada/

	* exp_ch3.adb (Expand_N_Object_Declaration): Use Typ local
	constant throughout, remove Ret_Obj_Typ local constant, rename
	Ref_Type into Acc_Typ in a couple of places, remove a useless call
	to Set_Etype, use a consistent checks suppression scheme, adjust
	comments for the sake of consistencty and alphabetize some local
	declarations.
	* exp_ch6.adb (Expand_Simple_Function_Return): Remove a couple of
	redundant local constants.