Remove extern crate tracing from numerous crates.

compiler/rustc_ast/src/lib.rs

@@ -20,9 +20,6 @@
 #![feature(negative_impls)]
 #![feature(stmt_expr_attributes)]
 
-#[macro_use]
-extern crate tracing;
-
 pub mod util {
     pub mod case;
     pub mod classify;

compiler/rustc_ast/src/util/literal.rs

@@ -8,6 +8,7 @@ use rustc_lexer::unescape::{
 use rustc_span::symbol::{kw, sym, Symbol};
 use rustc_span::Span;
 use std::{ascii, fmt, str};
+use tracing::debug;
 
 // Escapes a string, represented as a symbol. Reuses the original symbol,
 // avoiding interning, if no changes are required.