[clang] Use cast_or_null instead of cast (PR #93749)

[via cfe-commits]

cor3ntin wrote:

Can you provide a better description for the PR (such as "[clang] fix a crash with block captures inside a lambda" ) and an entry in clang/docs/ReleaseNotes.rst referencing the fixed issue? 

Thanks!

The change itself looks good except modulo @zyn0217's feedback

https://github.com/llvm/llvm-project/pull/93749