[clang] [clang] Reject using uninitialized reference in constant evaluation (PR #180923)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

If the committee changes the wording, sure, I guess we want something along these lines... but the diagnostic is bad.  Can we explicitly say that it's using the reference during its own initialization?

https://github.com/llvm/llvm-project/pull/180923