[clang] [Clang] Allow simpler visibility annotations when targeting win32 and mingw (PR #133699)

[Erich Keane via cfe-commits]

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@@ -6606,7 +6606,10 @@ void Sema::checkClassLevelDLLAttribute(CXXRecordDecl *Class) {
   if (ClassExported && !ClassAttr->isInherited() &&
       TSK == TSK_ExplicitInstantiationDeclaration &&
       !Context.getTargetInfo().getTriple().isWindowsGNUEnvironment()) {
-    Class->dropAttr<DLLExportAttr>();
+    if (auto *DEA = Class->getAttr<DLLExportAttr>()) {
+      Class->addAttr(DLLExportOnDeclAttr::Create(Context, DEA->getLoc()));
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erichkeane wrote:

I honestly would like this better if we just kept the `DLLExportAttr` and added a bool flag to it for `ignoredBecauseItsOnaDeclWhereItWouldntWorkForVariousReasons` in the `extra members` section.

https://github.com/llvm/llvm-project/pull/133699