Update rustc-rayon to 0.4

Author: cuviper

Cargo.toml

@@ -20,7 +20,7 @@ rayon = { version = "1.4.1", optional = true }
 
 # Internal feature, only used when building as part of rustc,
 # not part of the stable interface of this crate.
-rustc-rayon = { version = "0.3", optional = true }
+rustc-rayon = { version = "0.4", optional = true }
 
 [dependencies.hashbrown]
 version = "0.12"