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

[Erich Keane via cfe-commits]

erichkeane 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?

IIRC, Aaron is away this week (and was away last week for WG14). So we'll have to wait on him a bit.

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