[llvm] Add the 'initializes' attribute langref and support (PR #84803)

[Nikita Popov via llvm-commits]

nikic wrote:

A quick question before I look into this in more detail: Why do we need the dedicated ConstantRangeList class? I'd have expected to see this passed around as `ArrayRef<ConstantRange>` and stored as `TrailingObjects<ConstantRange>`.

I see it has some helpers to insert ranges in sorted order, but that seems more like something for the inference implementation rather than the attribute implementation.

https://github.com/llvm/llvm-project/pull/84803