[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 think there are some good parallel disscussions happening in the RFC, but despite their outcomes, we could probably update the PR to capture current behavior in those interesting cases.

I left a few comments along those lines, PTAL. 

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