rustc: Flag metadata compatible with multiple CGUs

It looks like the `OutputType::Metadata` kind in the compiler was
misclassified in rust-lang#38571 long ago by accident as incompatible with
codegen units and a single output file. This means that if you emit both
a linkable artifact and metadata it silently turns off multiple codegen
units unintentionally!

This commit corrects the situation to ensure that if `--emit metadata`
is used it doesn't implicitly disable multiple codegen units. This will
ensure we don't accidentally regress compiler performance when striving
to implement pipelined compilation!

src/librustc/session/config.rs

@@ -153,13 +153,12 @@ impl_stable_hash_via_hash!(OutputType);
 impl OutputType {
     fn is_compatible_with_codegen_units_and_single_output_file(&self) -> bool {
         match *self {
-            OutputType::Exe | OutputType::DepInfo => true,
+            OutputType::Exe | OutputType::DepInfo | OutputType::Metadata => true,
             OutputType::Bitcode
             | OutputType::Assembly
             | OutputType::LlvmAssembly
             | OutputType::Mir
-            | OutputType::Object
-            | OutputType::Metadata => false,
+            | OutputType::Object => false,
         }
     }