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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> We still want the final OK from Aaron, but he's at 35k feet right now on the way back from LLVM-Dev, so hopefully he'll have time to do a pass next week.

I'm back in the office now and digging out from under last week. I plan to call consensus (for or against) on the RFC this week as conversation seems to have died down on it, so if anyone has last minute arguments for/against the feature, please leave comments on Discourse!

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