This enables better integration with tools that handle GNAT's output.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* erroutc.ads (Get_Warning_Option): New function returning the
	option responsible for a warning if it exists.
	* erroutc.adb (Get_Warning_Option): Likewise.
	(Get_Warning_Tag): Rely on Get_Warning_Option when possible.
	* errout.adb (Output_JSON_Message): Emit option field.