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

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``````````bash
git-clang-format --diff bf631c63d01057321c070520a56a150ede32e47d 0cc4b77fce06730f6a6a8b242384036018ebfe79 -- clang/lib/Basic/Module.cpp clang/lib/Headers/__stddef_null.h clang/lib/Headers/__stddef_nullptr_t.h clang/lib/Headers/__stddef_offsetof.h clang/lib/Headers/__stddef_ptrdiff_t.h clang/lib/Headers/__stddef_rsize_t.h clang/lib/Headers/__stddef_size_t.h clang/lib/Headers/__stddef_unreachable.h clang/lib/Headers/__stddef_wchar_t.h clang/lib/Lex/ModuleMap.cpp clang/test/Modules/no-undeclared-includes-builtins.cpp clang/test/Modules/stddef.c
``````````

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<summary>
View the diff from clang-format here.
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``````````diff
diff --git a/clang/lib/Headers/__stddef_nullptr_t.h b/clang/lib/Headers/__stddef_nullptr_t.h
index d724b5cba1..7f3fbe6fe0 100644
--- a/clang/lib/Headers/__stddef_nullptr_t.h
+++ b/clang/lib/Headers/__stddef_nullptr_t.h
@@ -11,7 +11,8 @@
  * When -fbuiltin-headers-in-system-modules is set this is a non-modular header
  * and needs to behave as if it was textual.
  */
-#if !defined(_NULLPTR_T) || (__has_feature(modules) && !__building_module(_Builtin_stddef))
+#if !defined(_NULLPTR_T) ||                                                    \
+    (__has_feature(modules) && !__building_module(_Builtin_stddef))
 #define _NULLPTR_T
 
 #ifdef __cplusplus
diff --git a/clang/lib/Headers/__stddef_offsetof.h b/clang/lib/Headers/__stddef_offsetof.h
index 62c49c78bd..84172c6cd2 100644
--- a/clang/lib/Headers/__stddef_offsetof.h
+++ b/clang/lib/Headers/__stddef_offsetof.h
@@ -11,6 +11,7 @@
  * When -fbuiltin-headers-in-system-modules is set this is a non-modular header
  * and needs to behave as if it was textual.
  */
-#if !defined(offsetof) || (__has_feature(modules) && !__building_module(_Builtin_stddef))
+#if !defined(offsetof) ||                                                      \
+    (__has_feature(modules) && !__building_module(_Builtin_stddef))
 #define offsetof(t, d) __builtin_offsetof(t, d)
 #endif
diff --git a/clang/lib/Headers/__stddef_ptrdiff_t.h b/clang/lib/Headers/__stddef_ptrdiff_t.h
index 31ecc00b62..fd3c893c66 100644
--- a/clang/lib/Headers/__stddef_ptrdiff_t.h
+++ b/clang/lib/Headers/__stddef_ptrdiff_t.h
@@ -11,7 +11,8 @@
  * When -fbuiltin-headers-in-system-modules is set this is a non-modular header
  * and needs to behave as if it was textual.
  */
-#if !defined(_PTRDIFF_T) || (__has_feature(modules) && !__building_module(_Builtin_stddef))
+#if !defined(_PTRDIFF_T) ||                                                    \
+    (__has_feature(modules) && !__building_module(_Builtin_stddef))
 #define _PTRDIFF_T
 
 typedef __PTRDIFF_TYPE__ ptrdiff_t;
diff --git a/clang/lib/Headers/__stddef_rsize_t.h b/clang/lib/Headers/__stddef_rsize_t.h
index 9cb9c26611..dd433d40d9 100644
--- a/clang/lib/Headers/__stddef_rsize_t.h
+++ b/clang/lib/Headers/__stddef_rsize_t.h
@@ -11,7 +11,8 @@
  * When -fbuiltin-headers-in-system-modules is set this is a non-modular header
  * and needs to behave as if it was textual.
  */
-#if !defined(_RSIZE_T) || (__has_feature(modules) && !__building_module(_Builtin_stddef))
+#if !defined(_RSIZE_T) ||                                                      \
+    (__has_feature(modules) && !__building_module(_Builtin_stddef))
 #define _RSIZE_T
 
 typedef __SIZE_TYPE__ rsize_t;
diff --git a/clang/lib/Headers/__stddef_size_t.h b/clang/lib/Headers/__stddef_size_t.h
index 01d3a0c480..3dd7b1f379 100644
--- a/clang/lib/Headers/__stddef_size_t.h
+++ b/clang/lib/Headers/__stddef_size_t.h
@@ -11,7 +11,8 @@
  * When -fbuiltin-headers-in-system-modules is set this is a non-modular header
  * and needs to behave as if it was textual.
  */
-#if !defined(_SIZE_T) || (__has_feature(modules) && !__building_module(_Builtin_stddef))
+#if !defined(_SIZE_T) ||                                                       \
+    (__has_feature(modules) && !__building_module(_Builtin_stddef))
 #define _SIZE_T
 
 typedef __SIZE_TYPE__ size_t;
diff --git a/clang/lib/Headers/__stddef_unreachable.h b/clang/lib/Headers/__stddef_unreachable.h
index 1f9254f6bb..518580c92d 100644
--- a/clang/lib/Headers/__stddef_unreachable.h
+++ b/clang/lib/Headers/__stddef_unreachable.h
@@ -11,6 +11,7 @@
  * When -fbuiltin-headers-in-system-modules is set this is a non-modular header
  * and needs to behave as if it was textual.
  */
-#if !defined(unreachable) || (__has_feature(modules) && !__building_module(_Builtin_stddef))
+#if !defined(unreachable) ||                                                   \
+    (__has_feature(modules) && !__building_module(_Builtin_stddef))
 #define unreachable() __builtin_unreachable()
 #endif
diff --git a/clang/lib/Headers/__stddef_wchar_t.h b/clang/lib/Headers/__stddef_wchar_t.h
index 4239f619f0..bd69f63225 100644
--- a/clang/lib/Headers/__stddef_wchar_t.h
+++ b/clang/lib/Headers/__stddef_wchar_t.h
@@ -13,7 +13,8 @@
  * When -fbuiltin-headers-in-system-modules is set this is a non-modular header
  * and needs to behave as if it was textual.
  */
-#if !defined(_WCHAR_T) || (__has_feature(modules) && !__building_module(_Builtin_stddef))
+#if !defined(_WCHAR_T) ||                                                      \
+    (__has_feature(modules) && !__building_module(_Builtin_stddef))
 #define _WCHAR_T
 
 #ifdef _MSC_EXTENSIONS

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