diagnostics: modernize bison's syntax errors

We used to display the unexpected token first:

    $ bison foo.y
    foo.y:1.8-13: error: syntax error, unexpected %token, expecting character literal or identifier or <tag>
        1 | %token %token
          |        ^~~~~~

GCC uses a different format:

    $ gcc-mp-9 foo.c
    foo.c:1:5: error: expected identifier or '(' before ')' token
        1 | int()()()
          |     ^

and so does Clang:

    $ clang-mp-9.0 foo.c
    foo.c:1:5: error: expected identifier or '('
    int()()()
        ^
    1 error generated.

They display the unexpected token last (or not at all).  Also, they
don't waste width with "syntax error".  Let's try that.  It gives, for
the same example as above:

    $ bison foo.y
    foo.y:1.8-13: error: expected character literal or identifier or <tag> before %token
        1 | %token %token
          |        ^~~~~~

* src/complain.h, src/complain.c (syntax_error): New.
* src/parse-gram.y (yyreport_syntax_error): Use it.

src/complain.c

@@ -23,6 +23,7 @@
 #include "system.h"
 
 #include <argmatch.h>
+#include <ctype.h>
 #include <progname.h>
 #include <stdarg.h>
 #include <sys/stat.h>
@@ -263,6 +264,20 @@ severity_prefix (severity s)
 }
 
 
+static void
+severity_print (severity s, FILE *out)
+{
+  if (s != severity_disabled)
+    {
+      const char* style = severity_style (s);
+      begin_use_class (style, out);
+      fprintf (out, "%s:", severity_prefix (s));
+      end_use_class (style, out);
+      fputc (' ', out);
+    }
+}
+
+
 /*-----------.
 | complain.  |
 `-----------*/
@@ -442,16 +457,7 @@ error_message (const location *loc, int *indent, warnings flags,
         fprintf (stderr, "%*s", *indent - pos, "");
     }
 
-  const char* style = severity_style (sever);
-
-  if (sever != severity_disabled)
-    {
-      begin_use_class (style, stderr);
-      fprintf (stderr, "%s:", severity_prefix (sever));
-      end_use_class (style, stderr);
-      fputc (' ', stderr);
-    }
-
+  severity_print (sever, stderr);
   vfprintf (stderr, message, args);
   /* Print the type of warning, only if this is not a sub message
      (in which case the prefix is null).  */
@@ -465,7 +471,7 @@ error_message (const location *loc, int *indent, warnings flags,
         putc ('\n', stderr);
         flush (stderr);
         if (loc && !(flags & no_caret))
-          location_caret (*loc, style, stderr);
+          location_caret (*loc, severity_style (sever), stderr);
       }
   }
   flush (stderr);
@@ -587,3 +593,53 @@ duplicate_rule_directive (char const *directive,
                    _("previous declaration"));
   fixits_register (&second, "");
 }
+
+void
+syntax_error (location loc,
+              int argc, const char* argv[])
+{
+  if (complaint_status < status_complaint)
+    complaint_status = status_complaint;
+  assert (argc <= 5);
+  const char *format = NULL;
+  switch (argc)
+    {
+# define CASE(N, S)                         \
+      case N:                               \
+        format = S;                         \
+        break
+    default: /* Avoid compiler warnings. */
+      CASE (0, _("syntax error"));
+      CASE (1, _("unexpected %0$s"));
+      CASE (2, _("expected %1$s before %0$s"));
+      CASE (3, _("expected %1$s or %2$s before %0$s"));
+      CASE (4, _("expected %1$s or %2$s or %3$s before %0$s"));
+      CASE (5, _("expected %1$s or %2$s or %4$s or %5$s before %0$s"));
+# undef CASE
+    }
+  location_print (loc, stderr);
+  fputs (": ", stderr);
+  severity_print (severity_error, stderr);
+
+  while (*format)
+    if (format[0] == '%'
+        && isdigit (format[1])
+        && format[2] == '$'
+        && format[3] == 's'
+        && (format[1] - '0') < argc)
+      {
+        int i = format[1] - '0';
+        const char *style = i == 0 ? "unexpected" : "expected";
+        begin_use_class (style, stderr);
+        fputs (argv[i], stderr);
+        end_use_class (style, stderr);
+        format += 4;
+      }
+    else
+      {
+        fputc (*format, stderr);
+        ++format;
+      }
+  fputc ('\n', stderr);
+  location_caret (loc, "error", stderr);
+}