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");
 }