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

[Eli Friedman via cfe-commits]

efriedma-quic 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.

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