rustc: Flag metadata compatible with multiple CGUs

rustc: Flag metadata compatible with multiple CGUs

It looks like the `OutputType::Metadata` kind in the compiler was
misclassified in #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!