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

[Volodymyr Sapsai via cfe-commits]

vsapsai 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.

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