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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for this! The changes should come with a release note in `clang/docs/ReleaseNotes.rst` as well so users know about the new functionality.

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