Remove DepKind::CrateMetadata and pre-allocation of DepNodes

Remove much of the special-case handling around crate metadata
dependency tracking by replacing `DepKind::CrateMetadata` and the
pre-allocation of corresponding `DepNodes` with on-demand invocation
of the `crate_hash` query.

compiler/rustc_incremental/src/persist/load.rs

@@ -4,7 +4,6 @@ use rustc_data_structures::fx::FxHashMap;
 use rustc_hir::definitions::Definitions;
 use rustc_middle::dep_graph::{PreviousDepGraph, SerializedDepGraph, WorkProduct, WorkProductId};
 use rustc_middle::ty::query::OnDiskCache;
-use rustc_middle::ty::TyCtxt;
 use rustc_serialize::opaque::Decoder;
 use rustc_serialize::Decodable as RustcDecodable;
 use rustc_session::Session;
@@ -15,14 +14,6 @@ use super::file_format;
 use super::fs::*;
 use super::work_product;
 
-pub fn dep_graph_tcx_init(tcx: TyCtxt<'_>) {
-    if !tcx.dep_graph.is_fully_enabled() {
-        return;
-    }
-
-    tcx.allocate_metadata_dep_nodes();
-}
-
 type WorkProductMap = FxHashMap<WorkProductId, WorkProduct>;
 
 pub enum LoadResult<T> {