[IRGen][WMO] type descriptors generated for an imported Clang types are always known local declarations

This fixes a LNK4217 linker warning when building code on Windows

Author: hyp

include/swift/IRGen/Linking.h

@@ -1597,6 +1597,9 @@ class LinkEntity {
            getKind() == Kind::DispatchThunkAllocator ||
            getKind() == Kind::DispatchThunkDerivative;
   }
+  bool isNominalTypeDescriptor() const {
+    return getKind() == Kind::NominalTypeDescriptor;
+  }
 
   /// Determine whether this entity will be weak-imported.
   bool isWeakImported(ModuleDecl *module) const;

lib/IRGen/GenDecl.cpp

@@ -2412,6 +2412,22 @@ LinkInfo LinkInfo::get(const UniversalLinkageInfo &linkInfo,
       isKnownLocal = MD == swiftModule || MD->isStaticLibrary();
   }
 
+  if (!isKnownLocal && !isDefinition) {
+    if (auto *entityDC = entity.getDeclContextForEmission()) {
+      auto *entitySF = entityDC->getModuleScopeContext();
+      bool clangImportedEntity = isa<ClangModuleUnit>(entitySF);
+      // Nominal type descriptor for a type imported from a Clang module
+      // is always a local declaration as it's generated on demand. When WMO is off, it's emitted into
+      // the current file's object file. When WMO is on, it's emitted into
+      // one of the object files in the current module, and thus it's never
+      // imported from outside of the module.
+      if (clangImportedEntity &&
+          entity.isNominalTypeDescriptor()) {
+        isKnownLocal = true;
+      }
+    }
+  }
+
   bool weakImported = entity.isWeakImported(swiftModule);
   result.IRL = getIRLinkage(result.Name, linkInfo,
                             entity.getLinkage(isDefinition), isDefinition,