Fix #15130 FN passedByValue (nested anonymous union) - #8963

Test results for commit c594e9b (tools/test-my-pr.py, main compared to this PR):

Test: http://ec2-16-170-140-253.eu-north-1.compute.amazonaws.com/pr-8963/
packages: 100
Differing warnings: 0 (+0 -0)
Timing: +0.0% (http://ec2-16-170-140-253.eu-north-1.compute.amazonaws.com/pr-8963/2026-10-26_12-34-58-c594e9b063e0_timing.html)

_{Posted automatically by the cppcheck PR test runner. +N: warnings only with this PR, -N: warnings only with main. The AI review is written by Claude and can be wrong.}