Trait methods with body and >1 unnamed parameters are wrongly rejected by resolve

[petrochenkov]

Reproduces on stable/beta/nightly.

trait Tr {
    // error: identifier `` is bound more than once in this parameter list
    fn f(u8, u8) {}
}

fn main() {
}

[eddyb]

I expect it parses as <invalid>: u8, <invalid>: u8 and it might lower to HIR as _: u8, _: u8 although nothing should care at that point about the argument list.
However, resolve running on AST, it has to treat <invalid> (the empty identifier) as _.