[clang] [clang] Fix crash caused by undeduced auto return variable (PR #211124)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> Ok, I believe I've worked out what is going wrong. Please hold.

Ok, I think root cause fix is this: https://github.com/llvm/llvm-project/pull/211705

https://github.com/llvm/llvm-project/pull/211124