[WebAssembly] MC: Fix crash in getProvitionalValue on weak references

- Create helper function for resolving weak references.
- Add test that preproduces the crash.

Differential Revision: https://reviews.llvm.org/D37916

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313381 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed