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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I think this needs to work in C as well, and we should have tests for that too.  I see this as more useful in C than C++, and there isn't a good reason to restrict it.

Also, needs a release note.

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