Adds instance support for composite enums (#1857)

Fixes https://github.com/paritytech/polkadot-sdk/issues/1839

Currently, `composite_enum`s do not support pallet instances. This PR
allows the following:
```rust
	#[pallet::composite_enum]
	pub enum HoldReason<I: 'static = ()> {
		SomeHoldReason
	}
```

### Todo

- [x]  UI Test

substrate/frame/support/test/tests/construct_runtime_ui.rs

@@ -32,4 +32,5 @@ fn ui() {
 
 	let t = trybuild::TestCases::new();
 	t.compile_fail("tests/construct_runtime_ui/*.rs");
+	t.pass("tests/construct_runtime_ui/pass/*.rs");
 }

substrate/frame/support/test/tests/construct_runtime_ui/pass/composite_enum_instance.rs

@@ -0,0 +1,80 @@
+// This file is part of Substrate.
+
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#![cfg_attr(not(feature = "std"), no_std)]
+
+use frame_support::derive_impl;
+
+pub use pallet::*;
+
+#[frame_support::pallet(dev_mode)]
+pub mod pallet {
+	use frame_support::pallet_prelude::*;
+
+	// The struct on which we build all of our Pallet logic.
+	#[pallet::pallet]
+	pub struct Pallet<T, I = ()>(PhantomData<(T, I)>);
+
+	// Your Pallet's configuration trait, representing custom external types and interfaces.
+	#[pallet::config]
+	pub trait Config<I: 'static = ()>: frame_system::Config {}
+	
+	#[pallet::composite_enum]
+	pub enum HoldReason<I: 'static = ()> {
+		SomeHoldReason
+	}
+
+	#[pallet::composite_enum]
+	pub enum FreezeReason<I: 'static = ()> {
+		SomeFreezeReason
+	}
+
+	#[pallet::composite_enum]
+	pub enum SlashReason<I: 'static = ()> {
+		SomeSlashReason
+	}
+
+	#[pallet::composite_enum]
+	pub enum LockId<I: 'static = ()> {
+		SomeLockId
+	}
+}
+
+#[derive_impl(frame_system::config_preludes::TestDefaultConfig as frame_system::DefaultConfig)]
+impl frame_system::Config for Runtime {
+	type Block = Block;
+}
+
+pub type Header = sp_runtime::generic::Header<u64, sp_runtime::traits::BlakeTwo256>;
+pub type UncheckedExtrinsic = sp_runtime::generic::UncheckedExtrinsic<u64, RuntimeCall, (), ()>;
+pub type Block = sp_runtime::generic::Block<Header, UncheckedExtrinsic>;
+
+frame_support::construct_runtime!(
+	pub struct Runtime
+	{
+		// Exclude part `Storage` in order not to check its metadata in tests.
+		System: frame_system,
+		Pallet1: pallet,
+		Pallet2: pallet::<Instance2>,
+	}
+);
+
+impl pallet::Config for Runtime {}
+
+impl pallet::Config<pallet::Instance2> for Runtime {}
+
+fn main() {}