Mark DOMXPath as uncloneable

This never resulted in a working XPath object anyway, as trying to query
or evaluate anything resulted in an "Invalid XPath context" error.
Supporting th

Mark DOMXPath as uncloneable

This never resulted in a working XPath object anyway, as trying to query
or evaluate anything resulted in an "Invalid XPath context" error.
Supporting this is more trouble than it's worth, so just block the clone
operation.

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