Address review comments

- Add back `HirIdVec`, with a comment that it will soon be used.
- Add back `*_region` functions, with a comment they may soon be used.
- Remove `-Z borrowck_stats` completely. It didn't do anything.
- Remove `make_nop` completely.
- Add back `current_loc`, which is used by an out-of-tree tool.
- Fix style nits
- Remove `AtomicCell` with `cfg(parallel_compiler)` for consistency.

compiler/rustc_driver/src/lib.rs

@@ -157,10 +157,9 @@ impl<'a, 'b> RunCompiler<'a, 'b> {
         self
     }
     /// Used by RLS.
-    pub fn set_emitter(&mut self, emitter: Option<Box<dyn Write + Send>>) -> &mut Self
-    {
-           self.emitter = emitter;
-           self
+    pub fn set_emitter(&mut self, emitter: Option<Box<dyn Write + Send>>) -> &mut Self {
+        self.emitter = emitter;
+        self
     }
     /// Used by RLS.
     pub fn set_file_loader(