Mark __CLASS__ as experimental

Fixes #22288

Author: rwp0

pod/perldiag.pod

@@ -44,6 +44,12 @@ letter.
 
 =over 4
 
+=item __CLASS__ is experimental
+
+(S experimental::class) This warning is emitted if you use the C<__CLASS__>
+keyword of C<use feature 'class'>.  This keyword is currently
+experimental and its behaviour may change in future releases of Perl.
+
 =item accept() on closed socket %s
 
 (W closed) You tried to do an accept on a closed socket.  Did you forget

toke.c

@@ -8003,6 +8003,8 @@ yyl_word_or_keyword(pTHX_ char *s, STRLEN len, I32 key, I32 orig_keyword, struct
                     : newSVOP(OP_RUNCV, 0, &PL_sv_undef));
 
     case KEY___CLASS__:
+        Perl_ck_warner_d(aTHX_
+            packWARN(WARN_EXPERIMENTAL__CLASS), "__CLASS__ is experimental");
         FUN0(OP_CLASSNAME);
 
     case KEY_AUTOLOAD: