[WebAssembly] Error on relocations against undefined data symbols.
We can't (currently) meaningfully resolve certain types of relocations
against undefined data symbols. Previously when --allow-undefined was
used we were treating such relocation much like weak data symbols and
simply inserting zeros. This change turns such use cases in to an
This means that --allow-undefined is no longer effective for data
Differential Revision: https://reviews.llvm.org/D60882