Handle multi-byte bracket sequences in Ada lexer

As noted in an earlier patch, the Ada lexer does not handle multi-byte
bracket sequences.  This patch adds support for these for character
literals.  gdb does not generally seem to handle the Ada wide string
types, so for the time being these continue to be excluded -- but an
explicit error is added to make this more clear.

gdb/ada-valprint.c

@@ -277,7 +277,11 @@ ada_emit_char (int c, struct type *type, struct ui_file *stream,
 	fprintf_filtered (stream, "%c", c);
     }
   else
-    fprintf_filtered (stream, "[\"%0*x\"]", type_len * 2, c);
+    {
+      /* Follow GNAT's lead here and only use 6 digits for
+	 wide_wide_character.  */
+      fprintf_filtered (stream, "[\"%0*x\"]", std::min (6, type_len * 2), c);
+    }
 }
 
 /* Character #I of STRING, given that TYPE_LEN is the size in bytes