Merge pull request swiftlang#12353 from compnerd/dll-storage

IRGen: fix DLL storage for runtime functions

Author: compnerd

lib/IRGen/IRGenModule.cpp

@@ -472,7 +472,8 @@ llvm::Constant *swift::getRuntimeFn(llvm::Module &Module,
     fn->setCallingConv(cc);
 
     if (::useDllStorage(llvm::Triple(Module.getTargetTriple())) &&
-        (fn->getLinkage() == llvm::GlobalValue::ExternalLinkage ||
+        ((fn->getLinkage() == llvm::GlobalValue::ExternalLinkage &&
+          fn->isDeclaration()) ||
          fn->getLinkage() == llvm::GlobalValue::AvailableExternallyLinkage))
       fn->setDLLStorageClass(llvm::GlobalValue::DLLImportStorageClass);