patch 9.0.1355: no error when declaring a class twice

Problem:    No error when declaring a class twice. (Ernie Rael)
Solution:   Pass different flags when declaring the class. (closes #12057)
diff --git a/src/vim9class.c b/src/vim9class.c
index 4431640..a2b186e 100644
--- a/src/vim9class.c
+++ b/src/vim9class.c
@@ -230,7 +230,8 @@
     void
 ex_class(exarg_T *eap)
 {
-    int is_class = eap->cmdidx == CMD_class;  // FALSE for :interface
+    int	    is_class = eap->cmdidx == CMD_class;  // FALSE for :interface
+    long    start_lnum = SOURCING_LNUM;
 
     char_u *arg = eap->arg;
     int is_abstract = eap->cmdidx == CMD_abstract;
@@ -1097,8 +1098,9 @@
 	tv.v_type = VAR_CLASS;
 	tv.vval.v_class = cl;
 	is_export = class_export;
+	SOURCING_LNUM = start_lnum;
 	set_var_const(cl->class_name, current_sctx.sc_sid,
-					     NULL, &tv, FALSE, ASSIGN_DECL, 0);
+						       NULL, &tv, FALSE, 0, 0);
 	return;
     }