[Ada] Add precise subprogram identification to -fdiagnostics-format=json

[Pierre-Marie de Rodat <derodat@adacore.com>]

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This commit adds information allowing identification of the subprogram
surrounding the message emitted by gnat when using -gnatdJ along with
-fdiagnostics-format=json.

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

gcc/ada/

	* errout.adb (Write_JSON_Span): Add subprogram name to emitted
	JSON.

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diff --git a/gcc/ada/errout.adb b/gcc/ada/errout.adb
--- a/gcc/ada/errout.adb
+++ b/gcc/ada/errout.adb
@@ -2097,11 +2097,14 @@ package body Errout is
       --  Write Sptr as a JSON location, an object containing a file attribute,
       --  a line number and a column number.
 
-      procedure Write_JSON_Span (Span : Source_Span);
-      --  Write Span as a JSON span, an object containing a "caret" attribute
-      --  whose value is the JSON location of Span.Ptr. If Span.First and
-      --  Span.Last are different from Span.Ptr, they will be printed as JSON
+      procedure Write_JSON_Span (Error : Error_Msg_Object);
+      --  Write Error as a JSON span, an object containing a "caret" attribute
+      --  whose value is the JSON location of Error.Sptr.Ptr. If Sptr.First and
+      --  Sptr.Last are different from Sptr.Ptr, they will be printed as JSON
       --  locations under the names "start" and "finish".
+      --  When Include_Subprogram_In_Messages is true (-gnatdJ) an additional,
+      --  non-standard, attribute named "subprogram" will be added, allowing
+      --  precisely identifying the subprogram surrounding the span.
 
       -----------------------
       --  Is_Continuation  --
@@ -2162,7 +2165,8 @@ package body Errout is
       -- Write_JSON_Span --
       ---------------------
 
-      procedure Write_JSON_Span (Span : Source_Span) is
+      procedure Write_JSON_Span (Error : Error_Msg_Object) is
+         Span : constant Source_Span := Error.Sptr;
       begin
          Write_Str ("{""caret"":");
          Write_JSON_Location (Span.Ptr);
@@ -2177,6 +2181,11 @@ package body Errout is
             Write_JSON_Location (Span.Last);
          end if;
 
+         if Include_Subprogram_In_Messages then
+            Write_Str
+              (",""subprogram"":""" & Subprogram_Name_Ptr (Error.Node) & """");
+         end if;
+
          Write_Str ("}");
       end Write_JSON_Span;
 
@@ -2210,7 +2219,7 @@ package body Errout is
       --  Print message location
 
       Write_Str (",""locations"":[");
-      Write_JSON_Span (Errors.Table (E).Sptr);
+      Write_JSON_Span (Errors.Table (E));
 
       if Errors.Table (E).Optr /= Errors.Table (E).Sptr.Ptr then
          Write_Str (",{""caret"":");