util: Add Assume() identity function

Summary:

This is needed for #20138. Please refer to the added documentation for
motivation.

Backport of core#20255.

Depends on D9681.

Comes with a linter fix for an edge case where a macro defining
((void)(val)) would be linted as (()(val)).

Ref T1611.

Test Plan:
With and without debug:

ninja all check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1611

Differential Revision: https://reviews.bitcoinabc.org/D9680