[llvm] [llvm][ADT] Mark llvm::IntrusiveRefCntPtr with the warn_unused attribute (PR #211795)

[Balázs Benics via llvm-commits]

steakhal wrote:

I tried building llvm + clang + extra with this attribute and didn't produce any warnings - now that the linked PRs fixed some cases.

https://github.com/llvm/llvm-project/pull/211795