Backport stdarg/stddef -fbuiltin-headers-in-system-modules fix

[ian-twilightcoder]

We should backport #84127 to LLVM 18 because it's a regression from LLVM 17.

/cherry-pick f50d358

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Ian Anderson (ian-twilightcoder)

We should backport https://github.com//pull/84127 to LLVM 18 because it's a regression from LLVM 17.

/cherry-pick f50d358

[llvmbot]

/pull-request #85118