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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Fantastic, thank you! Could we merge it?

heh, thanks for the poke, I didn't remember you didn't have commit access. I've merged now.

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