[llvm] [ADT] Fix missing 'template' keyword (PR #98252)

[NAKAMURA Takumi via llvm-commits]

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@@ -460,8 +460,8 @@ namespace llvm {
 
     OwningArrayRef &operator=(OwningArrayRef &&Other) {
       delete[] this->data();
-      this->MutableArrayRef<T>::operator=(Other);
-      Other.MutableArrayRef<T>::operator=(MutableArrayRef<T>());
+      this->template MutableArrayRef<T>::operator=(Other);
+      Other.template MutableArrayRef<T>::operator=(MutableArrayRef<T>());
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chapuni wrote:

```
g++ (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0
```

https://github.com/llvm/llvm-project/pull/98252