Don't copy bytecode files into the incr. comp. cache.

It's no longer necessary now that bitcode is embedded into object files.

This change meant that `WorkProductFileKind::Bytecode` is no longer
necessary, which means that type is no longer necessary, which allowed
several places in the code to become simpler.

src/librustc_query_system/dep_graph/graph.rs

@@ -861,13 +861,7 @@ impl<K: DepKind> DepGraph<K> {
 pub struct WorkProduct {
     pub cgu_name: String,
     /// Saved files associated with this CGU.
-    pub saved_files: Vec<(WorkProductFileKind, String)>,
-}
-
-#[derive(Clone, Copy, Debug, RustcEncodable, RustcDecodable, PartialEq)]
-pub enum WorkProductFileKind {
-    Object,
-    Bytecode,
+    pub saved_files: Vec<String>,
 }
 
 #[derive(Clone)]

src/librustc_query_system/dep_graph/mod.rs

@@ -6,7 +6,6 @@ mod query;
 mod serialized;
 
 pub use dep_node::{DepNode, DepNodeParams, WorkProductId};
-pub use graph::WorkProductFileKind;
 pub use graph::{hash_result, DepGraph, DepNodeColor, DepNodeIndex, TaskDeps, WorkProduct};
 pub use prev::PreviousDepGraph;
 pub use query::DepGraphQuery;