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_interface/src/queries.rs

@@ -102,7 +102,7 @@ impl<'tcx> Queries<'tcx> {
     }
 
     fn session(&self) -> &Session {
-        &self.compiler.session()
+        self.compiler.session()
     }
 
     fn codegen_backend(&self) -> &dyn CodegenBackend {