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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

> I see that it is set and the reason the existing error is not emitted is because of the !Entry->isDeclaration() condition.

For non-lambda methods, the way this works it that we call GetOrCreateLLVMFunction for both methods... for the first method, the `!Entry->isDeclaration()` would be false, but for the second one, it would be true because we've emitted the definition of the first method.  What's different for lambda methods?

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