Prepare for the removal of TypeTag.UNKNOWN

The constant was removed as part of https://bugs.openjdk.org/browse/JDK-8339296

This is a minimal fix to avoid breaking for upcoming javac versions. I suspect this logic isn't actually necessary, we aren't supposed to process ASTs with errors and there have been a variety of improvements in how that's handled since this was added.

PiperOrigin-RevId: 690616927

Author: cushon

check_api/src/main/java/com/google/errorprone/util/ASTHelpers.java

@@ -1299,15 +1299,17 @@ public static boolean isVoidType(Type type, VisitorState state) {
   }
 
   private static final ImmutableSet<TypeTag> SUBTYPE_UNDEFINED =
-      Sets.immutableEnumSet(
-          TypeTag.METHOD, TypeTag.PACKAGE, TypeTag.UNKNOWN, TypeTag.ERROR, TypeTag.FORALL);
+      Sets.immutableEnumSet(TypeTag.METHOD, TypeTag.PACKAGE, TypeTag.ERROR, TypeTag.FORALL);
 
   /** Returns true if {@code erasure(s) <: erasure(t)}. */
   public static boolean isSubtype(Type s, Type t, VisitorState state) {
     if (s == null || t == null) {
       return false;
     }
-    if (SUBTYPE_UNDEFINED.contains(s.getTag()) || SUBTYPE_UNDEFINED.contains(t.getTag())) {
+    if (SUBTYPE_UNDEFINED.contains(s.getTag())) {
+      return false;
+    }
+    if (t == state.getSymtab().unknownType) {
       return false;
     }
     Types types = state.getTypes();