[Ada] Improve error messages for occurrence of GNAT extensions without -gnatX

[Ada] Improve error messages for occurrence of GNAT extensions without -gnatX

The error message issued for use of GNAT extension features without
specifying -gnatX (or pragma Extensions_Allowed) was confusing in the
presence of a pragma specifying a language version (such as "pragma
Ada_2022;"), because the pragma supersedes the switch.  The message is
improved by testing for use of such a pragma, plus use of pragma
Extensions_Allowed is now suggested, and several cases are changed to
call the common error procedure for flagging uses of extension features.

gcc/ada/

	* errout.ads (Error_Msg_GNAT_Extension): Add formal Loc and
	revise comment.
	* errout.adb (Error_Msg_GNAT_Extension): Condition message on
	the flag Ada_Version_Pragma, and add suggestion to use of pragma
	Extensions_Allowed in messages.
	* par-ch3.adb, par-ch5.adb, par-ch6.adb, par-ch11.adb,
	par-ch12.adb: Add actual Token_Ptr on calls to
	Error_Msg_GNAT_Extension.
	* par-ch4.adb: Change Error_Msg to Error_Msg_GNAT_Extension for
	error calls related to use of extension features.
	* sem_ch13.adb: Likewise.