Rollup merge of #133823 - estebank:issue-56328, r=petrochenkov

Use `cfg_attr_trace` in AST with a placeholder attribute for accurate suggestion

In rust-lang/rust#138515, we insert a placeholder attribute so that checks for attributes can still know about the placement of `cfg` attributes. When we suggest removing items with `cfg_attr`s (fix rust-lang/rust#56328) and make them verbose. We tweak the wording of the existing "unused `extern crate`" lint.

```
warning: unused `extern crate`
  --> $DIR/removing-extern-crate.rs:9:1
   |
LL | extern crate removing_extern_crate as foo;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unused
   |
note: the lint level is defined here
  --> $DIR/removing-extern-crate.rs:6:9
   |
LL | #![warn(rust_2018_idioms)]
   |         ^^^^^^^^^^^^^^^^
   = note: `#[warn(unused_extern_crates)]` implied by `#[warn(rust_2018_idioms)]`
help: remove the unused `extern crate`
   |
LL - #[cfg_attr(test, macro_use)]
LL - extern crate removing_extern_crate as foo;
   |
```

r? `@petrochenkov`

try-job: x86_64-gnu-aux

compiler/rustc_lint/src/lints.rs

@@ -3077,7 +3077,9 @@ pub(crate) struct ByteSliceInPackedStructWithDerive {
 #[derive(LintDiagnostic)]
 #[diag(lint_unused_extern_crate)]
 pub(crate) struct UnusedExternCrate {
-    #[suggestion(code = "", applicability = "machine-applicable")]
+    #[label]
+    pub span: Span,
+    #[suggestion(code = "", applicability = "machine-applicable", style = "verbose")]
     pub removal_span: Span,
 }