Add alolan exclusive form check

Marowak and Exeggutor are exempt due to their pre-evolutions not having
a form and past gen pkm evolving into the alolan variant.

PKHeX/Legality/Checks.cs

@@ -1365,6 +1365,9 @@ namespace PKHeX
                     }
                     break;
             }
+
+            if (pkm.Format >= 7 && pkm.AltForm != 0 && Legal.AlolanOriginForms.Contains(pkm.Species))
+            { AddLine(Severity.Invalid, "Form cannot be obtained for pre-Alola generation games.", CheckIdentifier.Form); return; }
             if (pkm.AltForm > 0 && new[] {Legal.BattleForms, Legal.BattleMegas, Legal.BattlePrimals}.Any(arr => arr.Contains(pkm.Species)))
             { AddLine(Severity.Invalid, "Form cannot exist outside of a battle.", CheckIdentifier.Form); return; }
 

PKHeX/Legality/Tables7.cs

@@ -268,8 +268,6 @@ namespace PKHeX
             new EncounterStatic { Species = 103, Form = 1, Level = 40, Relearn = new[]{000, 000, 000, 000}, Ability = 1, Location = -1, }, // Exeggutor-1
             new EncounterStatic { Species = 785, Form = 0, Level = 60, Relearn = new[]{000, 000, 000, 000}, Shiny = false, Ability = 1, Location = 030, }, // Tapu Koko
             new EncounterStatic { Species = 542, Form = 0, Level = 57, Relearn = new[]{382, 437, 014, 494}, Location = -1, }, // Leavanny
-
-
         };
         private static readonly EncounterTrade[] TradeGift_SM = // @ a\1\5\5
         {
@@ -293,6 +291,26 @@ namespace PKHeX
             0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, // Johto Balls
             0x1A, // Beast
         };
+        internal static readonly int[] AlolanOriginForms =
+        {
+            019, // Rattata
+            020, // Raticate
+            025, // Pikachu (Cosplay not transferrable)
+            026, // Raichu
+            027, // Sandshrew
+            028, // Sandslash
+            037, // Vulpix
+            038, // Ninetails
+            050, // Diglett
+            051, // Dugtrio
+            052, // Meowth
+            053, // Persian
+            074, // Geodude
+            075, // Graveler
+            076, // Golem
+            088, // Grimer
+            089, // Muk
+        };
         internal static readonly int[] PastGenAlolanNatives =
         {
             010, 011, 012, 019, 020, 021, 022, 025, 026, 027, 028, 035, 036, 037, 038, 039, 040, 041, 042, 046, 047, 050,