Merge QueryDescription into QueryConfig

`QueryDescription` has gone through a lot of refactoring and doesn't
make sense anymore.

compiler/rustc_query_impl/src/on_disk_cache.rs

@@ -1063,7 +1063,7 @@ pub fn encode_query_results<'a, 'tcx, CTX, Q>(
     query_result_index: &mut EncodedDepNodeIndex,
 ) where
     CTX: QueryContext + 'tcx,
-    Q: super::QueryDescription<CTX>,
+    Q: super::QueryConfig<CTX>,
     Q::Value: Encodable<CacheEncoder<'a, 'tcx>>,
 {
     let _timer = tcx