[Ada] Fix comments mentioning ancient flags related to objects references

[Ada] Fix comments mentioning ancient flags related to objects references

Flag May_Be_Modified under go a series of renamings between 1996 and
2002.  It was changed to Not_Assigned, then to Not_Source_Assigned and
finally to Never_Set_In_Source. Fix remaining references in comments.

gcc/ada/

	* sem_util.ads (Note_Possible_Modification): Fix occurrence of
	May_Be_Modified in comment.
	* sem_warn.ads (Check_Unset_Reference): Fix occurrence of
	Not_Assigned in comment.