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

[Ben Langmuir via cfe-commits]

================
@@ -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"}))
----------------
benlangmuir wrote:

Are we not considering the include of `stdarg.h` a use of `_Builtin_stdarg` 
here if it doesn't use the modular headers from `_Builtin_stdarg`?

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