Warn for type parameter defaults on `impl` blocks

[bluss]

If I understand correctly, type parameter defaults have no effect in the block impl<T = default> Foo<T> { }.

Just like in PR #30724, this should have a warning and then an error.

Example (playground)

use std::marker::PhantomData;

struct Foo<T> {
    x: PhantomData<T>,
}

impl<T = i32> Foo<T> {
    pub fn new() -> Self {
        Foo { x: PhantomData }
    }
}

fn main() {
    let x = Foo::new();  // error: unable to infer enough type information about `_`
}

cc @nikomatsakis

[nikomatsakis]

Argh, I guess I failed to cover those?

[nikomatsakis]

Nominating for triage.

[nikomatsakis]

triage: P-high