[llvm] Add comment and assert for CloneModule edge case (#67734)

CloneModule is not currently designed to handle un-materialized Modules,
for example one created via a lazy initializer like
getLazyBitcodeModule(). In this case we get a somewhat cryptic
segmentation fault without a clear path forward.

In this patch, we add a comment to inform CloneModule users of this
shortcoming, and an assert to test for empty function bodies before the
segmentation fault is triggered.