ada: Add quickfixes for -gnawk warnings

ada: Add quickfixes for -gnawk warnings

gcc/ada/ChangeLog:

	* errout.adb (Insert): New function to create an insertion fix.
	(Deletion): New function to create a deletion fix.
	* errout.ads: Likewise.
	* sem_warn.adb (Create_Add_Constant_Fix): New function to create
	a fix for adding a constant qualifier for a variable declaration.
	(Change_In_Out_To_In_Fix): New function to create a fix for
	convertinting an in out parameter direction to a an in direction.

Co-authored-by: Eric Botcazou <ebotcazou@adacore.com>