[WebAssembly] Error on relocations against undefined data symbols.

[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
error.

This means that `--allow-undefined` is no longer effective for data
symbols.

Fixes https://bugs.llvm.org/show_bug.cgi?id=40364

Differential Revision: https://reviews.llvm.org/D60882

llvm-svn: 358899