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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Oh, that makes sense... so the issue is generally with functions we emit lazily?

I'd say it's reasonable to emit an error if we have two definitions for the same symbol, even if we don't end up emitting them because they're deferred.  Lazy emission is an optimization, and we don't guarantee whether a particular odr-used function will be emitted.

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