[llvm] Don't rely on undefined behavior to store how a `User` object's allocation is laid out (PR #105714)

[Daniel Paoliello via llvm-commits]

dpaoliello wrote:

> For MLIR, here is the code to create and setup an Operation: https://github.com/llvm/llvm-project/blob/main/mlir/lib/IR/Operation.cpp#L114-L120

Yeah, that's one other option: instead of splitting this logic between `operator new` and the ctor, we roll them together into a static `Create` function. I considered that, but it seemed like it would cause huge churn throughout the code base.

https://github.com/llvm/llvm-project/pull/105714