single_use_lifetime should not be emitted for HRTB bounds

[kennytm]

Repro (on 1.28-stable to 1.31-nightly)

#![warn(single_use_lifetimes)]

pub trait Tfv<'a>: Sized {
    const FOO: &'a u8;
}

pub fn g<T: for<'a> Tfv<'a>>() {}

Unexpected lint result:

warning: lifetime parameter `'a` only used once
 --> src/lib.rs:7:17
  |
7 | pub fn g<T: for<'a> Tfv<'a>>() {}
  |                 ^^      -- ...is used only here
  |                 |
  |                 this lifetime...
  |

Unless we intend to support T: Tfv<'_>, there shouldn't be a lint here. cc #44752.

[Diggsey]

I've only ever seen this lint fire incorrectly...