[WebAssembly] Error on archives without a symbol index

This is fairly common with wasm since GNU ar (most likely the system ar)
doesn't support the wasm object format so user who don't override AR
will end up with archives without an index.  We don't want to silently
ignore this issue.

In the future we could choose to instead behave like the ELF backend and
read the symbols from each object file in the archive if they are all of
the same type.  However, error'ing out seem like a conservative approach
for now.

Fixes: PR42376

Differential Revision: https://reviews.llvm.org/D63739

llvm-svn: 364338