Invalid "a constant with a similar name exists: `_`" suggestion when a `const _` is in scope

[ollie27]

For example (playground):

const _: () = ();

fn main() {
    a
}

Errors with:

error[E0425]: cannot find value `a` in this scope
 --> src/main.rs:4:5
  |
1 | const _: () = ();
  | ----------------- similarly named constant `_` defined here
...
4 |     a
  |     ^ help: a constant with a similar name exists: `_`

The same also happens with use Trait as _ (playground):

pub use Iterator as _;

fn foo<T: A>(x: T) {}

error[E0405]: cannot find trait `A` in this scope
    --> src/lib.rs:3:11
     |
3    |   fn foo<T: A>(x: T) {}
     |             ^ help: a trait with a similar name exists: `_`

The diagnostics shouldn't suggest things named underscore.