[mlir] Add a copy constructor to FailureOr

[math-fehr]

Summary

The copy constructor was missing from FailureOr.

Note that I do not have commit access.

[math-fehr]

@rriddle, could you land this for me? I do not have commit access.
Also, I see that the build failed, but it seems to be unrelated to the patch. If this is a problem, could you restart the builds?

mlir/include/mlir/Support/LogicalResult.h

public:
   /// Allow constructing from a LogicalResult. The result *must* be a failure.
   /// Success results should use a proper instance of type `T`.
   FailureOr(LogicalResult result) {
     assert(failed(result) &&
            "success should be constructed with an instance of 'T'");
   }
   FailureOr() : FailureOr(failure()) {}
   FailureOr(T &&y) : Optional<T>(std::forward<T>(y)) {}
+  FailureOr(const T &y) : Optional<T>(y) {}
 
   operator LogicalResult() const { return success(this->hasValue()); }
 
 private:
   /// Hide the bool conversion as it easily creates confusion.
   using Optional<T>::operator bool;
   using Optional<T>::hasValue;
 };
 
 } // namespace mlir
 
 #endif // MLIR_SUPPORT_LOGICAL_RESULT_H