[clang] [Clang] Emit error for duplicate mangled names within a lambda (PR #107581)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Isn't this a bug in the mangler? I mean, it's better to print an error rather than silently miscompile, but this doesn't really solve the issue.

https://github.com/llvm/llvm-project/pull/107581