[libc] [llvm] [libc] unpoison memory returned by pipe syscall (PR #88942)

[Nick Desaulniers via llvm-commits]

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

+2 but cc @ramosian-glider since these changes are annoying to keep having to make. Are we "holding it wrong?"  For instance, our syscall wrapper here treats all arguments as inputs.

https://github.com/llvm/llvm-project/blob/main/libc/src/__support/OSUtil/linux/x86_64/syscall.h#L36-L40

But I suspect if we said the `pipefd` param was an ouput with `=D`, perhaps msan would know that the kernel is expected to overwrite the output and DTRT?

https://github.com/llvm/llvm-project/pull/88942