[llvm] [libc] [libc] Fix read under msan (PR #80203)

[Nick Desaulniers via llvm-commits]

https://github.com/nickdesaulniers approved this pull request.

cc @ramosian-glider @vitalybuka 

I assume this is right, but perhaps Alexander or Vitaly know if this isn't (and why)?

https://github.com/llvm/llvm-project/pull/80203