[clang] [Clang] Skip past code generation for unevaluated lambdas (PR #124572)

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cor3ntin wrote:

> I would expect that we should instead be marking this function not `isUsed` (or `isReferenced`? ) to determine whether this should be emitted, and I would expect that to get caught in our existing infrastructure for that.

And we do. But mangling happens before we make that decision
cf stacktrace here https://github.com/llvm/llvm-project/issues/82926#issuecomment-1963388189

https://github.com/llvm/llvm-project/pull/124572