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

[NAKAMURA Takumi via llvm-commits]

================
@@ -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>());
----------------
chapuni wrote:

This fails with Ubuntu-20.04 g++.

```
llvm/include/llvm/ADT/ArrayRef.h:463:40: error: expected ‘;’ before ‘::’ token
  463 |       this->template MutableArrayRef<T>::operator=(Other);
      |                                        ^~
      |                                        ;
llvm/include/llvm/ADT/ArrayRef.h:464:40: error: expected ‘;’ before ‘::’ token
  464 |       Other.template MutableArrayRef<T>::operator=(MutableArrayRef<T>());
      |                                        ^~
      |                                        ;
```

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