perf: Buffer stderr when writing json errors/warnings

Since `stderr` is unbuffered, writing out json messages actually take up
about ~10%/0.1s of the runtime of the `inflate` benchmark.

cc #64413

src/librustc_errors/json.rs

@@ -48,7 +48,7 @@ impl JsonEmitter {
         macro_backtrace: bool,
     ) -> JsonEmitter {
         JsonEmitter {
-            dst: Box::new(io::stderr()),
+            dst: Box::new(io::BufWriter::new(io::stderr())),
             registry,
             sm: source_map,
             pretty,