Parse ForeignItem type with generics and bounds

[dtolnay]

As tested by https://github.com/rust-lang/rust/blob/1836e3b42a5b2f37fd79104eedbe8f48a5afdee6/src/test/ui/parser/foreign-ty-syntactic-pass.rs.

extern "C" {
    type A: Ord;
    type A<'a> where 'a: 'static;
    type A<T: Ord> where T: 'static;
    type A = u8;
    type A<'a: 'static, T: Ord + 'static>: Eq + PartialEq where T: 'static + Copy = Vec<u8>;
}

Closes #763. Required for dtolnay/cxx#99.