[llvm] [msan] Unpoison indirect outputs for userspace using llvm.memset.* (PR #79924)

[Alexander Potapenko via llvm-commits]

ramosian-glider wrote:

LGTM in general.

But the downside here is that the compiler won't be able to optimize away repeated stores if they are transformed to memset calls.
Maybe introduce some threshold and only memset() sizes greater than, say, 64? (IIRC that's what `-ftrivial-auto-var-init` does)

https://github.com/llvm/llvm-project/pull/79924