Make OpOperand constructor private (NFC)

[mehdi_amini]

Summary

This is intended to avoid programming mistake where a temporary OpOperand is
created, for example:

for (OpOperand user : result.getUsers()) {

It can be confusing for the user, in particular since in MLIR most classes are intended to
be copied around by value while they have reference semantics.

mlir/include/mlir/IR/UseDefLists.h

private:
   void *impl;
 };
 } // namespace detail
 
 /// This class represents an operand of an operation. Instances of this class
 /// contain a reference to a specific `Value`.
 class OpOperand : public IROperand<OpOperand, detail::OpaqueValue> {
 public:
-  using IROperand<OpOperand, detail::OpaqueValue>::IROperand;
-
   /// Provide the use list that is attached to the given value.
   static IRObjectWithUseList<OpOperand> *getUseList(Value value);
 
   /// Return the current value being used by this operand.
   Value get() const;
 
   /// Set the operand to the given value.
   void set(Value value);
 
   /// Return which operand this is in the operand list of the User.
   unsigned getOperandNumber();
+
+private:
+  /// Keep the constructor private and accessible to the OperandStorage class
+  /// only to avoid hard-to-debug typo/programming mistakes.
+  friend class OperandStorage;
+  using IROperand<OpOperand, detail::OpaqueValue>::IROperand;
 };
 
 //===----------------------------------------------------------------------===//
 // ValueUseIterator
 //===----------------------------------------------------------------------===//
 
 /// An iterator class that allows for iterating over the uses of an IR operand
 /// type.