[clang] [clang][headers] Including stddef.h always redefines NULL (PR #99727)

[Ian Anderson via cfe-commits]

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@@ -49,7 +48,25 @@
 #define __need_rsize_t
 #endif
 #define __need_wchar_t
+#if !defined(__STDDEF_H) && !__building_module(_Builtin_stddef)
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ian-twilightcoder wrote:

clang/test/Modules/stddef.cpp shows how even if we've already had a full include of the entirety of `<stddef.h>`, we still `__need` everything. It also shows that you're right and `__building_module(_Builtin_stddef)` doesn't work, but `__has_feature(modules)` does.

https://github.com/llvm/llvm-project/pull/99727