Make HandleCycleError an enum instead of a macro-generated closure

- Add a `HandleCycleError` enum to rustc_query_system, along with a `handle_cycle_error` function
- Move `Value` to rustc_query_system, so `handle_cycle_error` can use it
- Move the `Value` impls from rustc_query_impl to rustc_middle. This is necessary due to orphan rules.

compiler/rustc_query_system/src/lib.rs

@@ -20,3 +20,7 @@ pub mod dep_graph;
 mod error;
 pub mod ich;
 pub mod query;
+mod values;
+
+pub use error::HandleCycleError;
+pub use values::Value;