Revert "Fixes clang warning about friend decl using struct instead of class."

This reverts commit 0385deac.

ConstructionTraits has been changed from class to struct in !361 (merged). Unfortunately --- committed/authored earlier but merged later than !361 (merged) as part of a bigger solver factory MR (!349 (merged)) --- commit 0385deac changed some occurrences of friend struct to friend class which made the compiler warning reappear. Reverting this commit should fix the compiler warnings once more and struct is then used everywhere.