Bitfields in unions are broken

[emilio]

Also followup from #739.

The following code:

union Foo {
  unsigned int bitfield: 1;
};

Will generate, with the stable rust option:

#[repr(C)]
#[derive(Debug, Copy)]
pub struct Foo {
    pub _bitfield_1: u8,
    pub bindgen_union_field: u32,
}

And with the unstable one:

pub union Foo {
    pub _bitfield_1: u8,
}

We don't have a special path for bitfields in unions, and we should force their offset to be at offset zero, and force alignment and all that.

It's presumably not ultra-hard to fix.