[C++20] [Modules] Don't set module ownership if the export decl is not valid.

[ChuanqiXu]

Summary

This is an alternative to D117093.

clang/lib/Sema/SemaModule.cpp

Decl *Sema::ActOnStartExportDecl(Scope *S, SourceLocation ExportLoc,
   if (auto *ED = getEnclosingExportDecl(D)) {
     Diag(ExportLoc, diag::err_export_within_export);
     if (ED->hasBraces())
       Diag(ED->getLocation(), diag::note_export);
   }
 
   CurContext->addDecl(D);
   PushDeclContext(S, D);
+  if (getCurrentModule() && getCurrentModule()->isModulePurview())
     D->setModuleOwnershipKind(Decl::ModuleOwnershipKind::VisibleWhenImported);
   return D;
 }
 
 static bool checkExportedDeclContext(Sema &S, DeclContext *DC,
                                      SourceLocation BlockStart);
 
 namespace {
 enum class UnnamedDeclKind {

clang/test/Modules/export-in-non-modules.cpp

+// RUN: %clang_cc1 -std=c++20 %s -fsyntax-only -verify
+export struct Unit { // expected-error {{export declaration can only be used within a module interface unit after the module declaration}}
+    bool operator<(const Unit&);
+};