[clang] Fix crash and improve diagnostics for void lambda returns (PR #188904)

[via cfe-commits]

https://github.com/Sirraide commented:

Code lg appart from the merge errors; also, the last CI run indicates that you’ll have to update some tests for CI to pass (which is expected considering that we’re changing some diagnostics here); see the CI comments above for more information

https://github.com/llvm/llvm-project/pull/188904