[clang] [clang][modules] giving the __stddef_ headers their own modules can cause redeclaration errors with -fbuiltin-headers-in-system-modules (PR #84127)

[Ian Anderson via cfe-commits]

ian-twilightcoder wrote:

> I'm not excited by the complexity we are moving toward with the builtin headers. But I don't have any alternatives.
> 
> Given the situation we are in, I think the change is ok. But I'd like someone else to look at it as it is tricky to reason about it. No blockers from me but want more people to review the change.

It's not my favorite. Hopefully Apple will be able to move off of `-fbuiltin-headers-in-system-modules` in a few years, including the Linux and Windows module maps in Swift. There may not be any other users, I haven't heard any other feedback about the new modules behavior yet. When `-fbuiltin-headers-in-system-modules` goes away, things become simpler than they were since modules were introduced.

https://github.com/llvm/llvm-project/pull/84127