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

[via cfe-commits]

higher-performance wrote:

@erichkeane I ended up just implementing a full-ish traversal since it didn't. I'm not entirely sure if it might miss any corner cases (like anonymous objects etc.), but it should handle any cases that practically come up. Please take another look.

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