[lld][WebAssembly] Fix for --relocatable and signature mismatches

This is a followup to https://reviews.llvm.org/D78779.

When signatures mismatch we create set of variant symbols.  Some of
the fields in these symbols were not be initialized correct.
Specifically we were seeing isUsedInRegularObj not being set correctly,
leading to the symbol not getting included in the symbol table
and a crash writing relections in --reloctable mode.

There is larger refactor due here, but this is a minimal change the
fixes the bug at hand.

Differential Revision: https://reviews.llvm.org/D79756