Allow TemplateSingleArgument on UDA (#3532)

Author: dkorpel

spec/attribute.dd

@@ -1005,7 +1005,7 @@ $(H2 $(LNAME2 uda, User-Defined Attributes))
 $(GRAMMAR
 $(GNAME UserDefinedAttribute):
     $(D @ $(LPAREN)) $(GLINK2 expression, ArgumentList) $(D $(RPAREN))
-    $(D @) $(GLINK_LEX Identifier)
+    $(D @) $(GLINK2 template, TemplateSingleArgument)
     $(D @) $(GLINK_LEX Identifier) $(D $(LPAREN)) $(GLINK2 expression, ArgumentList)$(OPT) $(D $(RPAREN))
     $(D @) $(GLINK2 template, TemplateInstance)
     $(D @) $(GLINK2 template, TemplateInstance) $(D $(LPAREN)) $(GLINK2 expression, ArgumentList)$(OPT) $(D $(RPAREN))