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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Ah haha I see, thanks! Looking forward to getting it sometime, that would definitely be helpful :)

FWIW, I think you're fine to ask for commit privs now: https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access

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