MozPromise most common use is to have an single or exclusive listener. By making the MozPromise generated by IPDL exclusive we can also use move semantics.
While at it, we also use move semantics for the ResponseRejectReason and via the callback's reject method so that the lambda used with the MozPromise::Then can be identical to the one used by the IPDL callback.
As it currently is, it provides no advantage over a copy as it's just an enum; however, this will facilitate future changes where it may not be.
Differential Revision: https://phabricator.services.mozilla.com/D13906
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extra : moz-landing-system : lando
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