single_use_lifetime should not be emitted for traits with a lifetime parameter and a single function using it

[kennytm]

Repro (on 1.27-stable to 1.31-nightly):

#![warn(single_use_lifetimes)]

pub trait Tfv<'a>: Sized {
    fn tfv(x: &'a str) -> Option<Self>;
    // fn shut_up_single_use_lifetimes(_: &'a ()) {} // uncomment to shut up the lint 😕 
}

Unexpected lint result:

warning: lifetime parameter `'a` only used once
 --> src/lib.rs:3:15
  |
3 | pub trait Tfv<'a>: Sized {
  |               ^^ this lifetime...
4 |     fn tfv(x: &'a str) -> Option<Self>;
  |                -- ...is used only here
  |

cc #44752

[jhpratt]

enums are also linted here as of 1.40