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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Can you try to explain in a bit more detail what this patch is trying to do?  From the test changes, it looks like all this does is change the diagnostics, and the new diagnostics don't look better.

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