Update error index generator to tracing

src/tools/error_index_generator/main.rs

 #![feature(rustc_private)]
 
-extern crate env_logger;
 extern crate rustc_ast;
+extern crate rustc_driver;
 extern crate rustc_span;
 
 use std::cell::RefCell;
@@ -282,7 +282,7 @@ fn parse_args() -> (OutputFormat, PathBuf) {
 }
 
 fn main() {
-    env_logger::init();
+    rustc_driver::init_env_logger("RUST_LOG");
     let (format, dst) = parse_args();
     let result = rustc_ast::with_default_session_globals(move || main_with_result(format, &dst));
     if let Err(e) = result {