Adapt cppcheck output parser to cppcheck v1.90

The GitLab runner machine previously ran Ubuntu 18.04, which featured
cppcheck 1.82. Some planned enhancements to the CI scripts required a newer
version of cppcheck, so we have upgraded it to Ubuntu 20.04, which has
cppcheck 1.90. However, the format of the diagnostics printed by cppcheck
has changed in 1.90, so our code that parsed them needs adapting to match.
1 job for cppcheck-1.90 in 4 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Generate
passed #38940
cross
generate

00:00:04