libsyntax: Don't strip types and lifetimes from single-segment paths in

[pcwalton]

patterns.

This breaks code like:

fn main() {
    match Some("foo") {
        None::<int> => {}
        Some(_) => {}
    }
}

Change this code to not contain a type error. For example:

fn main() {
    match Some("foo") {
        None::<&str> => {}
        Some(_) => {}
    }
}

Closes #16353.

[breaking-change]

r? @huonw