[clang] [clang] fix constexpr-unknown handling of self-references. (PR #132990)

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cor3ntin wrote:

> > Do you intend to backport this change to 20?
> 
> The weird behavior only shows up if you refer to a reference variable inside its own initializer, as far as I can tell. Maybe not worth backporting, even if it is a regression, because people shouldn't be writing code like that in the first place.

Fair enough, in that case, can you add a changelog entry? Thanks!

https://github.com/llvm/llvm-project/pull/132990