[llvm] [MSan] Fix va_arg handling on 32-bit machines (PR #211339)

[Egor Elchinov via llvm-commits]

Egor2001 wrote:

> Please reply to this message to confirm that you have read these policies, especially the LLVM AI Tool Use Policy, and that any AI tool usage has been noted in the PR description.

I've read the policies, but did not use any LLM-generated code while preparing this patch.

https://github.com/llvm/llvm-project/pull/211339