[cdac] Fix getting array function type in RuntimeTypeSystem.IsArrayMethod (#106512)

- Array method index should be the slot minus the number of virtuals, not the number of slots
- Constructor index should include 3, not just greater than

Author: elinor-fung

src/native/managed/cdacreader/src/Contracts/RuntimeTypeSystem_1.cs

@@ -678,14 +678,24 @@ public bool IsArrayMethod(MethodDescHandle methodDescHandle, out ArrayFunctionTy
             return false;
         }
 
-        int arrayMethodIndex = methodDesc.Slot - GetNumVtableSlots(GetTypeHandle(methodDesc.MethodTable));
-
+        // To get the array function index, subtract the number of virtuals from the method's slot
+        // The array vtable looks like:
+        //    System.Object vtable
+        //    System.Array vtable
+        //    type[] vtable
+        //    Get
+        //    Set
+        //    Address
+        //    .ctor        // possibly more
+        // See ArrayMethodDesc for details in coreclr
+        MethodTable methodTable = GetOrCreateMethodTable(methodDesc);
+        int arrayMethodIndex = methodDesc.Slot - methodTable.NumVirtuals;
         functionType = arrayMethodIndex switch
         {
             0 => ArrayFunctionType.Get,
             1 => ArrayFunctionType.Set,
             2 => ArrayFunctionType.Address,
-            > 3 => ArrayFunctionType.Constructor,
+            >= 3 => ArrayFunctionType.Constructor,
             _ => throw new InvalidOperationException()
         };
 
@@ -749,4 +759,11 @@ public bool IsILStub(MethodDescHandle methodDescHandle)
 
         return AsDynamicMethodDesc(methodDesc).IsILStub;
     }
+
+    private MethodTable GetOrCreateMethodTable(MethodDesc methodDesc)
+    {
+        // Ensures that the method table is valid, created, and cached
+        _ = GetTypeHandle(methodDesc.MethodTable);
+        return _methodTables[methodDesc.MethodTable];
+    }
 }