[clang] Add Clang attribute to ensure that fields are initialized explicitly (PR #102040)

[via cfe-commits]

higher-performance wrote:

Hi all, we've been waiting a few weeks for approval on this (from @AaronBallman I imagine), but I'm guessing there's a lack of bandwidth on the reviewers' sides. Is there a way we can move forward?

https://github.com/llvm/llvm-project/pull/102040