[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]

================
@@ -7,7 +7,7 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(NULL) || !__has_feature(modules)
+#if !defined(NULL) || !__building_module(_Builtin_stddef)
----------------
ian-twilightcoder wrote:

```
#if !defined(NULL) || !__has_feature(modules) || (__has_feature(modules) && 
!__building_module(_Builtin_stddef))
```
If `!__has_feature(modules)` then `__building_module(anything)` is `0`. So you 
can make these substitutions without changing the logic.
```
!__has_feature(modules)
!__has_feature(modules) && !__building_module(_Builtin_stddef)

!__has_feature(modules) || (__has_feature(modules) && 
!__building_module(_Builtin_stddef))
(!__has_feature(modules) && !__building_module(_Builtin_stddef)) || 
(__has_feature(modules) && !__building_module(_Builtin_stddef))

(!a && b) || (a && b)
b

(!__has_feature(modules) && !__building_module(_Builtin_stddef)) || 
(__has_feature(modules) && !__building_module(_Builtin_stddef))
!__building_module(_Builtin_stddef)
```

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