borrow_interior_mutable_const interacts poorly with the http crate

[sfackler]

The http crate defines constants for common header names, but any use of them triggers this lint:

use http::header::HOST;
use http::HeaderMap;

fn main() {
    let map = HeaderMap::new();
    println!("{}", map.contains_key(&HOST));
}

error: a const item with interior mutability should not be borrowed
 --> src/main.rs:6:38
  |
6 |     println!("{}", map.contains_key(&HOST));
  |                                      ^^^^
  |
  = note: #[deny(clippy::borrow_interior_mutable_const)] on by default
  = help: assign this const to a local or static variable, and use the variable here
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#borrow_interior_mutable_const

[jebrosen]

This looks like the same or similar underlying cause as #3962, through a few layers of enums and macros. HOST is pub const HOST: HeaderName { inner: Repr::Standard(StandardHeader::Host) }, where Repr::Standard is a non-interior-mutable variant. The Repr enum contains a Bytes in one of the variants (cc #5812).