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

[Erich Keane via cfe-commits]

erichkeane wrote:

> > I don't think this is a good idea, warning on EVERY use of this is incorrect in system headers. IT is going to result in a ton of we-want-to-be-suppressed-positives
> 
> Thanks @erichkeane. I think I fundamentally disagree with this. The entire intention of this attribute is to flag such a thing -- it's no different from calling `std::invoke(foo, x)` and expecting to get a diagnostic on _every_ invocation when the function is `void foo(int x, int y)`, especially when you consider that these are basically simulating named-parameters. Do you have a plausible use case in mind that makes you believe people would actually want to suppress the warning in system headers?

Hmm... I just spent a while looking at the attribute more and changed how I was thinking about this problem, and I'm less confident in my above statement.  SINCE this is competely opt-in we could presumably document this better to mean "we mean this even if it is in the nitty-gritty of the standard library".  Typically these sorts of attributes don't really mean that/are intended for use in violations by the 'users' code.  Of course, `construct-at` and `invoke`/etc all make this somewhat more complicated of a differentiation.

My biggest 'plausible' concern is cases where the system header needs to make irrelevant/unused temporaries for the purposes of various operations along the way, which still DO the right thing but are wrong along the way.  BUT I also wonder how much sympathy I have for those cases when the author of the type has opted into this.

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