diagnostics: display suggested update after the caret-info

This commit adds the suggestion in green, on the line below the
caret-and-tildes.

    foo.y:1.1-14: warning: deprecated directive: '%error-verbose', use '%define parse.error verbose' [-Wdeprecated]
        1 | %error-verbose
          | ^~~~~~~~~~~~~~
          | %define parse.error verbose

The current approach, with location_caret_suggestion, is fragile:
there's a protocol of calls to the complain functions which is strict.
We should rather have a richer structure describing the diagnostics,
including with submessages such as the suggestions, passed in the end
to the routines in charge of formatting and printing them.

* src/location.h, src/location.c (location_caret_suggestion): New.
* src/complain.c (deprecated_directive): Use it.
* tests/diagnostics.at, tests/input.at: Adjust expectations.

src/complain.c

@@ -546,17 +546,16 @@ bison_directive (location const *loc, char const *directive)
 void
 deprecated_directive (location const *loc, char const *old, char const *upd)
 {
-  if (feature_flag & feature_caret)
-    complain (loc, Wdeprecated,
-              _("deprecated directive, use %s"),
-              quote_n (1, upd));
-  else
-    complain (loc, Wdeprecated,
-              _("deprecated directive: %s, use %s"),
-              quote (old), quote_n (1, upd));
-  /* Register updates only if -Wdeprecated is enabled.  */
   if (warning_is_enabled (Wdeprecated))
-    fixits_register (loc, upd);
+    {
+      complain (loc, Wdeprecated,
+                _("deprecated directive: %s, use %s"),
+                quote (old), quote_n (1, upd));
+      if (feature_flag & feature_caret)
+        location_caret_suggestion (*loc, upd, stderr);
+      /* Register updates only if -Wdeprecated is enabled.  */
+      fixits_register (loc, upd);
+    }
 }
 
 void

src/location.c

@@ -470,6 +470,19 @@ location_caret (location loc, const char *style, FILE *out)
   }
 }
 
+void
+location_caret_suggestion (location loc, const char *s, FILE *out)
+{
+  const char *style = "insertion";
+  fprintf (out, "      | %*s",
+           loc.start.column - 1 - caret_info.skip + (caret_info.skip ? 3 : 0),
+           "");
+  begin_use_class (style, out);
+  fputs (s, out);
+  end_use_class (style, out);
+  putc ('\n', out);
+}
+
 bool
 location_empty (location loc)
 {

src/location.h

@@ -125,6 +125,10 @@ void caret_free (void);
    with the color STYLE.  */
 void location_caret (location loc, const char* style, FILE *out);
 
+/* Display a suggestion of replacement for LOC with S.  To call after
+   location_caret.  */
+void location_caret_suggestion (location loc, const char *s, FILE *out);
+
 /* Return -1, 0, 1, depending whether a is before, equal, or
    after b.  */
 static inline int