[lld][WebAssembly] Fix crash with `-pie` without `--allow-undefined`

`shouldImport` was not returning true in PIC mode even though out
assumption elsewhere (in Relocations.cpp:scanRelocations) is that we
don't report undefined symbols in PIC mode today.  This was resulting
functions that were undefined and but also not imported which hits an
assert later on that all functions have valid indexes.

Differential Revision: https://reviews.llvm.org/D101716