[llvm] [ADT] Initialize function_ref::callable (PR #216686)

[Mehdi Amini via llvm-commits]

joker-eph wrote:

Copying my question from the original PR:

> Can you clarify the path to failure: how is this used without being set in our codebase? From what I can see from the class invariant, this will be left unset only when callback is nullptr. And only callback is used for the checks in this class.
>
> Maybe copying a default-constructed (or nullptr constructed) function_ref would read this field uninitialized?

https://github.com/llvm/llvm-project/pull/216686