Rollup merge of #147762 - weihanglo:rustdoc-depinfo-stdout, r=fmease

feat(rustdoc): `--emit=depinfo` output to stdout via `-`

rustdoc's `--emit=depinfo` flag now supports using `-` to write the output to stdout,
aligning with rustc's behavior.

This will fix <https://github.com/rust-lang/rust/issues/147649>.

### How to review

* The first commit demonstrates that `rustdoc --emit=depinfo=-` hasn't yet supported emitting to stdout.
* The second implements it and the diff shows how the behavior changes.

compiler/rustc_session/src/config.rs

@@ -1111,7 +1111,7 @@ impl Input {
     }
 }
 
-#[derive(Clone, Hash, Debug, HashStable_Generic, PartialEq, Encodable, Decodable)]
+#[derive(Clone, Hash, Debug, HashStable_Generic, PartialEq, Eq, Encodable, Decodable)]
 pub enum OutFileName {
     Real(PathBuf),
     Stdout,