[WebAssembly] MC: Fix for address taken aliases

[WebAssembly] MC: Fix for address taken aliases

Previously, taking the address for an alias would result in:
 "Symbol not found in table index space"

Increase test coverage for weak aliases.

This code should be more efficient too as it avoids building
the `IsAddressTaken` set.

Differential Revision: https://reviews.llvm.org/D41510

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@321384 91177308-0d34-0410-b5e6-96231b3b80d8