Move metadata object generation for dylibs to the linker code

This deduplicates some code between codegen backends and may in the
future allow adding extra metadata that is only known at link time.

compiler/rustc_session/src/session.rs

@@ -215,13 +215,6 @@ pub struct Session {
     pub invocation_temp: Option<String>,
 }
 
-#[derive(PartialEq, Eq, PartialOrd, Ord)]
-pub enum MetadataKind {
-    None,
-    Uncompressed,
-    Compressed,
-}
-
 #[derive(Clone, Copy)]
 pub enum CodegenUnits {
     /// Specified by the user. In this case we try fairly hard to produce the