Use Vec<u8> for LOCAL_STD{OUT,ERR} instead of dyn Write.

It was only ever used with Vec<u8> anyway. This simplifies some things.

- It no longer needs to be flushed, because that's a no-op anyway for
  a Vec<u8>.

- Writing to a Vec<u8> never fails.

- No #[cfg(test)] code is needed anymore to use `realstd` instead of
  `std`, because Vec comes from alloc, not std (like Write).

compiler/rustc_interface/src/util.rs

@@ -148,9 +148,7 @@ pub fn setup_callbacks_and_run_in_thread_pool_with_globals<F: FnOnce() -> R + Se
 
     let main_handler = move || {
         rustc_span::with_session_globals(edition, || {
-            if let Some(stderr) = stderr {
-                io::set_panic(Some(stderr.clone()));
-            }
+            io::set_panic(stderr.clone());
             f()
         })
     };
@@ -188,9 +186,7 @@ pub fn setup_callbacks_and_run_in_thread_pool_with_globals<F: FnOnce() -> R + Se
             // on the new threads.
             let main_handler = move |thread: rayon::ThreadBuilder| {
                 rustc_span::SESSION_GLOBALS.set(session_globals, || {
-                    if let Some(stderr) = stderr {
-                        io::set_panic(Some(stderr.clone()));
-                    }
+                    io::set_panic(stderr.clone());
                     thread.run()
                 })
             };