Apparently rust bindgen uses a hack which assumes all specializations of
`UniquePtr` have the basic layout and use a pointer member with an empty
deleter.
This incorrect assumption unfortunately comes up for UniqueFileHandle,
which replaces the pointer type using the deleter with a file handle
helper type.
As this fails to build with RUST_BINDGEN, this patch takes the same
approach as bug 1802320, and just disables building these types when
building headers for bindgen.
This does not fix the general issue of bindgen making incorrect
assumptions about the layout of UniquePtr with non-default deleters.
Ideally, all non-default deleters should be made opaque.
Differential Revision: https://phabricator.services.mozilla.com/D223628
bb85d44899