impl_header_lifetime_elision errors can mention in-band lifetimes when that feature is disabled

[scottmcm]

https://play.rust-lang.org/?gist=63df9129c4f3d2c76f2620acc38d0e57&version=nightly&edition=2018

fn foo<'a>(x: &str, y: &'a str) {}

trait Foo<T> {}
impl<'a> Foo<&str> for &'a str {}

The fn elision is totally fine, but the translation to an impl header gives

error[E0688]: cannot mix in-band and explicit lifetime definitions
 --> src/lib.rs:4:14
  |
4 | impl<'a> Foo<&str> for &'a str {}
  |      --      ^ in-band lifetime definition here
  |      |
  |      explicit lifetime definition here

This 1) shouldn't be an error, and 2) shouldn't mention "in-band lifetime"s even if it is.

cc impl_header_lifetime_elision tracking issue, #15872

(This feature is on-by-default in 2018, but not yet stable in 2015.)