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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

Just about there IMO, still have an open on the two diagnostic things, but otherwise only a few nit-like things (but please update the tests, it makes it much easier to review/update in the future)

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