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_const_eval/src/const_eval/machine.rs

@@ -200,7 +200,7 @@ impl<'mir, 'tcx: 'mir> CompileTimeEvalContext<'mir, 'tcx> {
                 &caller
                     .file
                     .name
-                    .for_scope(&self.tcx.sess, RemapPathScopeComponents::DIAGNOSTICS)
+                    .for_scope(self.tcx.sess, RemapPathScopeComponents::DIAGNOSTICS)
                     .to_string_lossy(),
             ),
             u32::try_from(caller.line).unwrap(),