ada: Allow warnings with explain code

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.