This enables destructors for tagged unions in cbindgen, implemented in:
* https://github.com/eqrion/cbindgen/pull/333
Which allow us to properly generate a destructor for the cbindgen-generated
StyleBasicShape (which now contains an OwnedSlice).
For now, we still use the glue code to go from Box<BasicShape> to
UniquePtr<BasicShape>. But that will change in the future when we generate even
more stuff and remove all the glue.
I could add support for copy-constructor generation to cbindgen for tagged
enums, but I'm not sure if it'll end up being needed, and copy-constructing
unions in C++ is always very tricky.
Differential Revision: https://phabricator.services.mozilla.com/D29769