Replace use of TSystem::ConcatFileName with PrependPathName.

[pcanal]

This will remove spurious memory allocation and deallocation

[FabioGhattasPolimi]

I'm currently working on it
If it's not a problem, I would like to take care of this issue

[dpiparo]

Thanks. Feel free to reach out during the process if you have any doubts, this could help the final review and integration of the changes. Do not forget unit tests :)

[silverweed]

Once this is solved I would propose marking ConcatFileName as R__DEPRECATED for 6.38

[AasmaGupta]

Hi! Can I work on this issue as a beginner contribution?