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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Sure, it makes sense to print a diagnostic for lambdas.

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I'm having a bit of trouble understanding the way the new code is structured.  What makes the definition of lambda call operators special here?  Do we not call GetOrCreateLLVMFunction with IsForDefinition set?

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