Rollup merge of #54488 - zackmdavis:and_the_case_of_the_unused_crate, r=estebank

in which we include attributes in unused `extern crate` suggestion spans

![unused_extern](https://user-images.githubusercontent.com/1076988/45921698-50243e80-be6f-11e8-930a-7b2a33b4935c.png)

Resolves #54400.

r? @estebank

src/test/ui/rust-2018/issue-54400-unused-extern-crate-attr-span.fixed

@@ -0,0 +1,24 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// aux-build:edition-lint-paths.rs
+// run-rustfix
+// compile-flags:--extern edition_lint_paths --cfg blandiloquence
+// edition:2018
+
+#![deny(rust_2018_idioms)]
+#![allow(dead_code)]
+
+// The suggestion span should include the attribute.
+
+
+//~^ ERROR unused extern crate
+
+fn main() {}

src/test/ui/rust-2018/issue-54400-unused-extern-crate-attr-span.rs

@@ -0,0 +1,25 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// aux-build:edition-lint-paths.rs
+// run-rustfix
+// compile-flags:--extern edition_lint_paths --cfg blandiloquence
+// edition:2018
+
+#![deny(rust_2018_idioms)]
+#![allow(dead_code)]
+
+// The suggestion span should include the attribute.
+
+#[cfg(blandiloquence)] //~ HELP remove it
+extern crate edition_lint_paths;
+//~^ ERROR unused extern crate
+
+fn main() {}

src/test/ui/rust-2018/issue-54400-unused-extern-crate-attr-span.stderr

@@ -0,0 +1,18 @@
+error: unused extern crate
+  --> $DIR/issue-54400-unused-extern-crate-attr-span.rs:22:1
+   |
+LL | / #[cfg(blandiloquence)] //~ HELP remove it
+LL | | extern crate edition_lint_paths;
+   | | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-
+   | |________________________________|
+   |                                  help: remove it
+   |
+note: lint level defined here
+  --> $DIR/issue-54400-unused-extern-crate-attr-span.rs:16:9
+   |
+LL | #![deny(rust_2018_idioms)]
+   |         ^^^^^^^^^^^^^^^^
+   = note: #[deny(unused_extern_crates)] implied by #[deny(rust_2018_idioms)]
+
+error: aborting due to previous error
+