Allow additional trait bounds for #[pallet::constant] (#9050)

* Allow additional trait bounds for constants

* Add ui test for constants with additional trait bounds

* Update trait constant ui test

* Import syn::Error

* Use reference instead of cloning

* Add extra invalid bound ui test

* Out or order valid bounds

* Fix ui test

* Fix ui test

* Apply review suggestion about error message

substrate/frame/support/test/tests/pallet_ui/pass/trait_constant_valid_bounds.rs

@@ -0,0 +1,29 @@
+#[frame_support::pallet]
+mod pallet {
+	use frame_support::pallet_prelude::*;
+	use frame_system::pallet_prelude::BlockNumberFor;
+
+	#[pallet::config]
+	pub trait Config: frame_system::Config {
+		#[pallet::constant]
+		type U: Get<u32>;
+
+		#[pallet::constant]
+		type V: Get<u32> + From<u16>;
+
+		#[pallet::constant]
+		type W: From<u16> + Get<u32>;
+	}
+
+	#[pallet::pallet]
+	pub struct Pallet<T>(core::marker::PhantomData<T>);
+
+	#[pallet::hooks]
+	impl<T: Config> Hooks<BlockNumberFor<T>> for Pallet<T> {}
+
+	#[pallet::call]
+	impl<T: Config> Pallet<T> {}
+}
+
+fn main() {
+}