[Swift] Moves VTs from enums to structs to prevent empty enum generation

Moves VTs from enums to structs to prevent empty enum generation, which would usually cause a compilation error.

tests/swift/Tests/Flatbuffers/empty_vtable_generated.swift

@@ -0,0 +1,67 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+// swiftlint:disable all
+// swiftformat:disable all
+
+#if canImport(Common)
+import Common
+#endif
+
+import FlatBuffers
+
+public struct DataModel_A: FlatBufferTable, FlatbuffersVectorInitializable, Verifiable, ObjectAPIPacker {
+
+  static func validateVersion() { FlatBuffersVersion_25_12_19() }
+  public var __buffer: ByteBuffer! { return _accessor.bb }
+  private var _accessor: Table
+
+  private init(_ t: Table) { _accessor = t }
+  public init(_ bb: ByteBuffer, o: Int32) { _accessor = Table(bb: bb, position: o) }
+
+  private struct VT {
+  }
+
+  public static func startA(_ fbb: inout FlatBufferBuilder) -> UOffset { fbb.startTable(with: 1) }
+  public static func endA(_ fbb: inout FlatBufferBuilder, start: UOffset) -> Offset { let end = Offset(offset: fbb.endTable(at: start)); return end }
+  public static func createA(
+    _ fbb: inout FlatBufferBuilder
+  ) -> Offset {
+    let __start = DataModel_A.startA(&fbb)
+    return DataModel_A.endA(&fbb, start: __start)
+  }
+
+  public func unpack() -> DataModel_AT {
+    return DataModel_AT(self)
+  }
+  public static func pack(_ builder: inout FlatBufferBuilder, obj: inout DataModel_AT?) -> Offset {
+    guard var obj = obj else { return Offset() }
+    return pack(&builder, obj: &obj)
+  }
+
+  public static func pack(_ builder: inout FlatBufferBuilder, obj: inout DataModel_AT) -> Offset {
+    let __root = DataModel_A.startA(&builder)
+    return DataModel_A.endA(&builder, start: __root)
+  }
+
+  public static func verify<T>(_ verifier: inout Verifier, at position: Int, of type: T.Type) throws where T: Verifiable {
+    var _v = try verifier.visitTable(at: position)
+    _v.finish()
+  }
+}
+
+extension DataModel_A: Encodable {
+  public func encode(to encoder: Encoder) throws {
+  }
+}
+
+public class DataModel_AT: NativeObject {
+
+
+  public init(_ _t: borrowing DataModel_A) {
+  }
+
+  public init() {
+  }
+
+  public func serialize() -> ByteBuffer { return serialize(type: DataModel_A.self) }
+
+}