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

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

@AaronBallman this seems to have progressed much further after your review and is currently pending on input from you. Could you please do a round of review here/in the RFC? (Or ask someone else from the community to address your concerns and be a reviewer instead, not sure if that's more useful)

@cor3ntin and also the same question. Could you take another look to see if your concerns were addressed?

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