Disable util::Result copying and assignment
Summary:
It disables copying of util::Result objects because unnecessary copies are inefficient and not possible after #25665, which makes util::Result object move-only. It disables the assignment operator and replaces it with an Update() method, because #25665 adds more information to util::Result objects (warning and error messages and failure values) and having an assignment operator that overwrites data instead of merging it would make it easy to accidentally erase existing information while trying to assign new information.
Backport of core#29906.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D17970