[clang] [clang] return type not correctly deduced for discarded lambdas (PR #153921)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

@cor3ntin This is not actually correct as it fails to handle the not all paths return case. The problem here is that I don't really know just how the full deduction path handles that? (I'll continue to prod around but this area is not familiar to me)

https://github.com/llvm/llvm-project/pull/153921