Hide the UnsafeCell content of opaque C++ types in regard to unwind safety

[dtolnay]

Opaque C++ types used to be automatically RefUnwindSafe prior to #1370, which is a fine default. This PR fixes a regression that manifests like this:

error[E0277]: the type `UnsafeCell<PhantomData<()>>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
   --> tests/test.rs:389:61
    |
389 |     let _ref_unwind_safe = |c: &ffi::C| panic::catch_unwind(|| inspect(c));
    |                                         ------------------- ^^^^^^^^^^^^^ `UnsafeCell<PhantomData<()>>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
    |                                         |
    |                                         required by a bound introduced by this call
    |
    = help: within `cxx_test_suite::ffi::C`, the trait `RefUnwindSafe` is not implemented for `UnsafeCell<PhantomData<()>>`, which is required by `{closure@tests/test.rs:389:61: 389:63}: UnwindSafe`
note: required because it appears within the type `cxx::opaque::SyncUnsafeCell<PhantomData<()>>`
   --> src/opaque.rs:24:8
    |
24  | struct SyncUnsafeCell<T>(UnsafeCell<T>);
    |        ^^^^^^^^^^^^^^
note: required because it appears within the type `cxx::private::Opaque`
   --> src/opaque.rs:16:12
    |
16  | pub struct Opaque {
    |            ^^^^^^
note: required because it appears within the type `cxx_test_suite::ffi::C`
   --> tests/ffi/lib.rs:98:14
    |
98  |         type C;
    |              ^
    = note: required for `&cxx_test_suite::ffi::C` to implement `UnwindSafe`
note: required because it's used within this closure
   --> tests/test.rs:389:61
    |
389 |     let _ref_unwind_safe = |c: &ffi::C| panic::catch_unwind(|| inspect(c));
    |                                                             ^^
note: required by a bound in `std::panic::catch_unwind`
   --> $RUSTUP_HOME/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:344:40
    |
344 | pub fn catch_unwind<F: FnOnce() -> R + UnwindSafe, R>(f: F) -> Result<R> {
    |                                        ^^^^^^^^^^ required by this bound in `catch_unwind`