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

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov commented:

I hope folks are ok with me chiming in as a reviewer for this.
I've left quite a few comments in the RFC and is also supportive of landing this change and happy to invest into supporting it going forward inside our team.

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