`--with-derive-custom-struct` doesn't apply to `__BindgenBitfieldUnit`

Invoking bindgen with --with-derive-custom-struct .*=Foo does not add #[derive(Foo)] to __BindgenBitfieldUnit.

I'd like all structs (& unions) to be marked with the derive macro, as it only implements the trait when all fields also implement it.

My current solution is to just manually implement the trait for __BindgenBitfieldUnit, which is fine, but not stable when you decide to rework how bitfields are represented.

Start by tracing how --with-derive-custom-struct .*=Foo is applied to generated structs, then inspect generation of __BindgenBitfieldUnit. Add a regression test covering this bitfield type and verify that the requested derive appears on it; done means the custom derive applies consistently to the generated unit.