[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:

We should consider this for LLVM 18, it's a regression from https://reviews.llvm.org/D159064

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