Recover from | -> => typo

[estebank]

It is not uncommon when reworking match arms or adding a new variant to have in-progress code that looks like

match foo {
    Enum::Variant1(x) => {
    Enum::Variant1(x) => {
    }
    _ => {}
}

by copying the existing line when trying to get to

match foo {
    Enum::Variant2(x) |
    Enum::Variant1(x) => {
    }
    _ => {}
}

The parser could detect cases like these and avoid further parse errors after this point.