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

[Erich Keane via cfe-commits]

erichkeane 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.

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