Set dllimport on Objective C ivar offsets

This commit ensures that offsets for instance variables are marked with `dllimport` if the interface to which they belong have this attribute.

clang/lib/CodeGen/CGObjCGNU.cpp

@@ -1698,12 +1698,17 @@ class CGObjCGNUstep2 : public CGObjCGNUstep {
   }
   llvm::Value *EmitIvarOffset(CodeGenFunction &CGF,
                               const ObjCInterfaceDecl *Interface,
-                              const ObjCIvarDecl *Ivar) override {
-    const std::string Name = GetIVarOffsetVariableName(Ivar->getContainingInterface(), Ivar);
+                              const ObjCIvarDecl *Ivar) override {                            
+    const ObjCInterfaceDecl *ContainingInterface = Ivar->getContainingInterface();
+    const std::string Name = GetIVarOffsetVariableName(ContainingInterface, Ivar);
     llvm::GlobalVariable *IvarOffsetPointer = TheModule.getNamedGlobal(Name);
-    if (!IvarOffsetPointer)
+    if (!IvarOffsetPointer) {
       IvarOffsetPointer = new llvm::GlobalVariable(TheModule, IntTy, false,
               llvm::GlobalValue::ExternalLinkage, nullptr, Name);
+      if (Ivar->getAccessControl() != ObjCIvarDecl::Private
+         && Ivar->getAccessControl() != ObjCIvarDecl::Package)
+        CGM.setGVProperties(IvarOffsetPointer, ContainingInterface);
+    }
     CharUnits Align = CGM.getIntAlign();
     llvm::Value *Offset =
         CGF.Builder.CreateAlignedLoad(IntTy, IvarOffsetPointer, Align);

clang/test/CodeGenObjC/dllstorage.m

@@ -151,7 +151,7 @@ id f(Q *q) {
 
 // CHECK-IR-DAG: @"OBJC_IVAR_$_M._ivar" = external dllimport global i32
 
-// CHECK-NF-DAG: @"__objc_ivar_offset_M._ivar.@" = external global i32
+// CHECK-NF-DAG: @"__objc_ivar_offset_M._ivar.@" = external dllimport global i32
 
 int g(void) {
   @autoreleasepool {