util/check: Add CHECK_NONFATAL identity function and a NONFATAL_UNREACHABLE macro

Summary: This is a backport of core#24812

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13096