mergeFrom

Because the way we currently handle parallel parsing in TranslationManager.parseParallel, we end up with multiple GlobalScope objects, one for each TranslationUnitDeclaration. In the end, we need to merge all these different scopes into one final global scope. To be somewhat consistent with the behaviour of TranslationManager.parseSequentially, we assign the last translation unit declaration we see to the AST node of the GlobalScope. This is not completely ideal, but the best we can do for now.