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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

(You shouldn't need to special-case lambdas; any use of the type needs the appropriate mangling, except maybe the function declaration itself.)

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