[NFC] [C++20] [Modules] Refactoring b6c7177 to make it not

dependent on f109b10

Given
f109b10#commitcomment-113477829,
we need to revert f109b10. So it will be better to make this patch not
dependent on f109b10 as much as possible.

clang/lib/AST/ASTContext.cpp

@@ -11927,7 +11927,9 @@ bool ASTContext::DeclMustBeEmitted(const Decl *D) {
     return false;
 
   // Variables in other module units shouldn't be forced to be emitted.
-  if (VD->isInAnotherModuleUnit())
+  auto *VM = VD->getOwningModule();
+  if (VM && VM->getTopLevelModule()->isModulePurview() &&
+      VM->getTopLevelModule() != getCurrentNamedModule())
     return false;
 
   // Variables that can be needed in other TUs are required.