Support _ for const inference with const generics

[varkor]

Currently, users may specify _ in place of a type: the compiler will then suggest the intended type. This ought to work for consts too, but currently does not (see #79557).

fn foo<const N: usize, const K: usize>(data: [u32; N]) -> [u32; K] {
    [0; K]
}

fn main() {
    let a = foo::<_, 2>([0, 1, 2]);
}

The difficulty is that the intended use of _ (as either a type or const) is ambiguous until type checking, so runs into the same problems as #66615 and #77773.