ambigous_pattern lint

Ref: https://github.com/rust-lang/rfcs/issues/2272

I would like an ambiguous_pattern lint that either:

1. requires the pattern to have ::
2. requires the pattern to have @

Those 2 requirements make it easy to avoid ambiguous patterns, for example:

match x {
    C => {/*is this C a binding, or did you forget to import a constant?*/},
    x => {/*is this x a lowercase constant or did you mean to make a binding?*/},
}

Instead, write it as:

match x {
    m::C => {/*got constant C*/},
    x @ _ => {/*got unknown*/},
}

The issue names no implementation files or tests. Start by reviewing Clippy's existing pattern-lint entry points and the referenced Rust RFC issue, then determine how the proposed lint should distinguish qualified constants from bindings. Done means enforcing either :: or @ for the ambiguous patterns shown in the examples.