Struct initializers allow missing fields

[brson]

struct BuildData {
    foo: int,
    bar: ~int
}

fn main() {
    let foo = BuildData {
        foo: 0
    };

    debug!("%?", foo.bar);
}

This will crash

[brson]

There is code for this in check_struct_or_variant_fields that doesn't seem to be working. There aren't tests.

[brson]

This is more or less the same bug as #3486, different manifestations.