Lowering constants is currently not really compatible with unions. In
this commit, constant lowering is drastically overhauled: instead of
playing nice and generating SPIR-V constant representations for everything
directly, we're just going to treat globals as an untyped bag of bytes (
or rather, SPIR-V 32-bit words), which we cast to the desired type at
usage. This is similar to how Rust generates constants in its LLVm backend.