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

[via cfe-commits]

higher-performance wrote:

@AaronBallman I... just noticed that the name `clang::requires_explicit_initialization` is subtly different from `clang::require_constant_initialization`. (`requires` vs. `require`.) Is it worth making it consistent before people start using it? I can submit a PR quickly if so.

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