Implement missing members assist fills in consts with default value

[crumblingstatue]

trait Foo {
    const C: i32 = 42;

    fn missing();
}

struct Bar;

impl Foo for Bar {
    
}

The assist generates the following:

impl Foo for Bar {
    const C: i32 = 42; // Unnecessary, there is already a default

    fn missing() {
        todo!()
    }
}

rust-analyzer version: rust-analyzer version: 0.3.1212-standalone

rustc version: rustc 1.66.0-nightly (4a1467723 2022-09-23)