[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:

> Still need to check the code but
> 
> > Make the __stddef_ headers be non-modular in -fbuiltin-headers-in-system-modules and restore them back to not respecting their header guards. Still define the header guards though.
> 
> sounds quite questionable.

They were non-modular prior to https://reviews.llvm.org/D159064

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