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

On Apple platforms, some of the stddef.h types are also declared in
system headers. In particular NULL has a conflicting declaration in
<sys/_types/_null.h>. When that's in a different module from
<__stddef_null.h>, redeclaration errors can occur.

Make the \_\_stddef_ headers be non-modular in
-fbuiltin-headers-in-system-modules and restore them back to not
respecting their header guards. Still define the header guards though.
__stddef_max_align_t.h was in _Builtin_stddef_max_align_t prior to the
addition of _Builtin_stddef, and it needs to stay in a module because
struct's can't be type merged. __stddef_wint_t.h didn't used to have a
module, but leave it in it current module since it doesn't really belong
to stddef.h.

1 files changed, 3 insertions, 4 deletions

diff --git a/clang/lib/Basic/Module.cpp b/clang/lib/Basic/Module.cpp
index 9f597dc..256365d 100644
--- a/clang/lib/Basic/Module.cpp
+++ b/clang/lib/Basic/Module.cpp
@@ -301,10 +301,9 @@ bool Module::directlyUses(const Module *Requested) {
     if (Requested->isSubModuleOf(Use))
       return true;
 
-  // Anyone is allowed to use our builtin stdarg.h and stddef.h and their
-  // accompanying modules.
-  if (Requested->getTopLevelModuleName() == "_Builtin_stdarg" ||
-      Requested->getTopLevelModuleName() == "_Builtin_stddef")
+  // Anyone is allowed to use our builtin stddef.h and its accompanying modules.
+  if (Requested->fullModuleNameIs({"_Builtin_stddef", "max_align_t"}) ||
+      Requested->fullModuleNameIs({"_Builtin_stddef_wint_t"}))
     return true;
 
   if (NoUndeclaredIncludes)