Fix clippy::needless_borrow in the compiler

`x clippy compiler -Aclippy::all -Wclippy::needless_borrow --fix`.

Then I had to remove a few unnecessary parens and muts that were exposed
now.

compiler/rustc_session/src/config.rs

@@ -857,7 +857,7 @@ impl OutFileName {
     pub fn as_path(&self) -> &Path {
         match *self {
             OutFileName::Real(ref path) => path.as_ref(),
-            OutFileName::Stdout => &Path::new("stdout"),
+            OutFileName::Stdout => Path::new("stdout"),
         }
     }