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

[via cfe-commits]

higher-performance wrote:

I updated the PR to change the attribute name from `explicit` to `explicit_init` and to clarify the error message that initializations must be explicit.

@AaronBallman are we good to move forward?

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