Make OpOperand constructor private (NFC)

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.

Differential Revision: https://reviews.llvm.org/D81815