Change the way explain codes are handled to generate the command for the
user to get the explanations, as this was not working for warnings.

gcc/ada/

	* errout.adb (Error_Msg_Internal): Remove call to
	Prescan_Message on the special continuation for the explain code
	command, as this does not play well with the setting of global
	variables for the message (like its status as a warning or not).
	Instead, set directly the global variables regarding content of
	the message in terms of special characters.