I'm interested in what y'all think about the second issue; it's something
I've run into previously when thinking about expanding NEW_EXPR.

One problem with (b) is that using it requires the frontend to know about
passing by invisible reference.  However, in the cases where we pass by
invisible reference because TREE_ADDRESSABLE is set on the type, it's
already controlled by the frontend, so that doesn't seem like a big issue.
For cases where we would pass by invisiref for other reasons, doing a
bitwise copy is just inefficient, and we could leave it up to the optimizer
to fix that up.