libc++ doesn't yet have the code for this, but libc++abi does. Adding
the switch to libc++ since the flag in libc++abi is
_LIBCXX_DYNAMIC_FALLBACK, not _LIBCXXABI_DYNAMIC_FALLBACK, and it
will be needed here as well.
zygoloid and nbjoerg got me pointed in the right direction on this. Looks like the user didn't have a key function defined for one of their classes, which was actually the root of the problem.