This patch implements the -gnatw.d switch to activate tagging of warning
messages. With this switch set, warning messages will have a tag at the
end which is one of:

   [-gnatw?]               ? in a .. z
   [-gnatw.?]              ? in a .. z
   [enabled by default]

So, similar to the tags emitted by GCC for other languages.

The patch enables the general mechanism (using new insertion tags ??
and ?x?). So far only a few messages have been tagged, but eventually
we will tag as many warning messages as possible. The following source
program is compiled with -gnatj.p.d -gnatj70:

     1. function warndoc (a, b, c : integer) return integer is
     2.    x : string := %abc%;
                         |
        >>> warning: use of "%" is an obsolescent feature (RM
            J.2(4)), use """ instead [-gnatwj]

     3. begin
     4.    if b > 0 then
           |
        >>> warning: "return" statement missing following this
            statement, Program_Error may be raised at run time
            [enabled by default]

     5.       return warndoc (b, a, c);
                     |
        >>> warning: actuals for this call may be in wrong order
            [-gnatw.p]

     6.    end if;
     7. end;

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

2013-01-02  Robert Dewar  <dewar@adacore.com>

	* err_vars.ads (Warning_Doc_Switch): New flag.
	* errout.adb (Error_Msg_Internal): Implement new warning flag
	doc tag stuff (Set_Msg_Insertion_Warning): New procedure.
	* errout.ads: Document new insertion sequences ?? ?x? ?.x?
	* erroutc.adb (Output_Msg_Text): Handle ?? and ?x? warning doc
	tag stuff.
	* erroutc.ads (Warning_Msg_Char): New variable.
	(Warn_Chr): New field in error message object.
	* errutil.adb (Error_Msg): Set Warn_Chr in error message object.
	* sem_ch13.adb: Minor reformatting.
	* warnsw.adb: Add handling for -gnatw.d and -gnatw.D
	(Warning_Doc_Switch).
	* warnsw.ads: Add handling of -gnatw.d/.D switches (warning
	doc tag).