CppCheck: small `AnalyzerInformation` usage cleanup

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I was trying to make the object local instead of a class member but as it is being used by reportErr() that is unfortunately not possible.

Even I did not achieve that goal the usage checks got more explicit instead of relying on Settings::buildDir all over the code making things easier to read.

[firewave]

I think I might need to add some builddir-related tests first.

[firewave]

The fact that CppCheck object owns the Settings object complicates things. That also needs to be resolved first it seems...