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

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

FWIW, what I originally had in mind was to actually not have separate ConstantRange and ConstantRangeList attribute kinds, and only have the latter -- the existing ConstantRange kind would just be a list with a single element.

The upside of doing that is that we have less attribute kinds to deal with :) The downside is that the representation for the single-attribute case becomes less efficient (because we have to explicitly store the size 1).

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