Add syntactic to whitelist (#4597)

Part of #4579

Adds the `syntactic` attribute to the whitelist, and checks that it is
only applied to simplification rules.

Notably, we still need to:
- Check that the arguments actually refer to existing clauses
- Tag the underlying clauses and re-construct the attribute at the end
of the compilation pipeline because the `requires` clauses may be
manipulated during compilation

k-frontend/src/main/java/org/kframework/compile/checks/CheckAtt.java

@@ -60,6 +60,7 @@ private void checkRule(Rule rule) {
     checkNonExecutable(rule);
     checkSimplification(rule);
     checkSymbolic(rule);
+    checkSyntactic(rule);
   }
 
   private void checkProduction(Production prod) {
@@ -151,6 +152,14 @@ private void checkSymbolic(Rule rule) {
     }
   }
 
+  private void checkSyntactic(Rule rule) {
+    if (rule.att().contains(Att.SYNTACTIC()) && !rule.att().contains(Att.SIMPLIFICATION())) {
+      errors.add(
+          KEMException.compilerError(
+              "syntactic attribute is only supported on simplification rules."));
+    }
+  }
+
   private void checkHookedSortConstructors(Production prod) {
     if (prod.sort().equals(Sorts.KItem())) {
       return;

k-frontend/src/main/scala/org/kframework/attributes/Att.scala

@@ -416,6 +416,8 @@ object Att {
       onlyon3[Module, Production, Rule],
       KeyRange.WholePipeline
     )
+  final val SYNTACTIC =
+    Key.builtin("syntactic", KeyParameter.Required, onlyon[Rule], KeyRange.WholePipeline)
   final val TOKEN = Key.builtin(
     "token",
     KeyParameter.Forbidden,