Introduce a new debug switch -gnatdJ to prepend the name of the enclosing
entity (subprogram, task, ...) relevant for a given warning or style message.
This can be useful in the context of integrating these messages in other tools,
e.g. CodePeer or gnatcheck.

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

2017-09-08  Arnaud Charlet  <charlet@adacore.com>

	* exp_intr.adb (Append_Entity_Name): Move to ...
	* sem_util.ads, sem_util.adb: ... here to share it.
	(Subprogram_Name): New subprogram, to compute the name of the enclosing
	subprogram/entity.
	* errutil.adb (Error_Msg): Fill new field Node.
	* erroutc.ads (Subprogram_Name_Ptr): New.
	(Error_Msg_Object): New field Node.
	* erroutc.adb (dmsg, Output_Msg_Text): Take new field Node into account.
	* errout.adb (Error_Msg): New variant with node id parameter.
	Fill new parameter Node when emitting messages. Revert previous
	changes for Include_Subprogram_In_Messages.
	* sem_ch5.adb (Check_Unreachable_Code): Supply Node parameter when
	generating warning message.