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

[Ben Langmuir via cfe-commits]

benlangmuir wrote:

>> I'm not excited by the complexity we are moving toward with the builtin headers. But I don't have any alternatives.
> When -fbuiltin-headers-in-system-modules goes away, things become simpler than they were since modules were introduced.

Even for the case with `-fbuiltin-headers-in-system-modules` things aren't any more complex than they were before https://reviews.llvm.org/D159064 except for the fundamental complexity of having two modes to build in, and hopefully -fbuiltin-headers-in-system-modules goes away.

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