util: Make Assume() usable as unary expression

Summary:
Assume shouldn't behave different at the call site depending on build flags. Currently compilation fails if it is used as expression. Fix that by using the lambda approach from Assert() without the assert().

This is a backport of core#21317

Test Plan: ninja && ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10855