[clang] Diagnosis for constexpr constructor not initializing a union member Resolves #46689 (PR #81042)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

Thank you the PR.

Your summary is blank but should describe the problem you are trying to solve, how the PR solves it and should at the end link to any llvm issues this relates to.

Having solid summaries is critical since this is normally what one sees when using `git log` and folks should be able to understand the change without having to goto the PR itself or open up the issue that it solves. 

Also please add `[Clang][Sema]` to the front of your title so folks can quickly see what area it applies to.

Without having looked at the fix in detail, this will also require a release note.

https://github.com/llvm/llvm-project/pull/81042