Abolish QueryVTable in favour of more assoc items on QueryConfig

This may introduce additional mono _but_ may help const fold things
better and especially may help not constructing a `QueryVTable` anymore
which is cheap but not free.

compiler/rustc_query_system/src/query/mod.rs

@@ -12,7 +12,7 @@ pub use self::caches::{
 };
 
 mod config;
-pub use self::config::{QueryConfig, QueryVTable};
+pub use self::config::{HashResult, QueryConfig, TryLoadFromDisk};
 
 use crate::dep_graph::DepKind;
 use crate::dep_graph::{DepNodeIndex, HasDepContext, SerializedDepNodeIndex};