[libunwind] [Libunwind] Try to fix msan failures (PR #120013)

[Alexander Richardson via cfe-commits]

https://github.com/arichardson requested changes to this pull request.

We shouldn't need to initialize these structures to silence sanitizer errors. I had a draft pr last year to fix this properly but forgot about it since other things came up: https://github.com/llvm/llvm-project/pull/67860

I'd prefer something like that. Alternatively I can update my change once I'm back in the office in January.

https://github.com/llvm/llvm-project/pull/120013