[llvm] [LLVM][ADT] Add helper class for working with caches (PR #171008)

[Victor Chernyakin via llvm-commits]

localspook wrote:

Oh that's interesting, yeah, this `defer` is basically a simplified version of that paper's `elide`. Which is interesting, since we seem to have completely different use cases in mind! The paper calls it `elide` because it only seems concerned with enabling copy elision. My use case doesn't involve any elision, I'm only concerned with deferring work until it's needed, so I called it `defer`. I feel like neither of those names captures both use cases...

https://github.com/llvm/llvm-project/pull/171008