[clang] Include [[clang::require_explicit_initialization]] warnings in system headers (PR #141133)

[via cfe-commits]

higher-performance wrote:

Interesting point! A few thoughts:

- Are system headers really any different from non-system headers in that case? It feels like it might be an orthogonal issue we should address in a separate PR.
- Do you mean SFINAE contexts rather than merely unevaluated contexts (?)
- How would I detect if a given `Expr` (say, an `InitListExpr`) is unevaluated? Is that easy or would it require a huge rework of the implementation?

https://github.com/llvm/llvm-project/pull/141133