util, refactor: Add UNIQUE_NAME helper macro

Summary:
This change replaces repetitive code with a helper macro.

This is a backport of core#24355

Test Plan:

cmake .. -GNinja -DCMAKE_BUILD_TYPE=Debug -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++

ninja all check-all`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14890