[clang] [clang][Sema] Fix crash on decomposition decl missing initializer (PR #210151)

[Patryk Stefanski via cfe-commits]

patrykstefanski wrote:

> Just needs a release note, else this LGTM.

22ca91ce27af31f579a8a770a4d584f47bcc607f (PR #127924) introduced the crash and landed a week ago on the dev branch. The new diagnostic didn't ship yet. Should we still add 'crash fix' to release notes or rather skip it?

https://github.com/llvm/llvm-project/pull/210151