[clang] [Clang][Sema] fix crash in codegen stage when an lambda expression declared in an unevaluated context (PR #80802)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

A few suggestions, but I REALLY would like @cor3ntin to take a final pass, it mostly LGTM.

https://github.com/llvm/llvm-project/pull/80802