[Ada] Improve error messages for dot notation when -gnatX not used

[Ada] Improve error messages for dot notation when -gnatX not used

gcc/ada/

	* einfo.ads (Direct_Primitive_Operations): Update the doc to
	indicate that this field is used for all types now.
	* sem_ch4.adb (Try_Object_Operation): Add parameter
	Allow_Extensions set to True to pretend that extensions are
	allowed.
	* sem_ch4.ads: Same.
	* sem_ch6.adb: Do not require Extensions_Allowed.
	* sem_ch8.adb (Find_Selected_Component): Remove duplicate
	"where" in comment.  Improve the error messages regarding use of
	prefixed calls.