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

[via cfe-commits]

cor3ntin wrote:

Let me chat with @AaronBallman and @erichkeane on Monday with this.

This feels like a very narrow fix for a widespread issue. 
 - Presumably, we don't want to warn if library implementers (mis)use std::construct_at internally
 - There are _tons_ of other warnings that are silenced.

So I think that either
 - We need a better heuristics to hide warnings than "comes from a system header"
 - We should add the ShowInSystemHeader flags to a lot of warnings, not just this one. 

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