Use std::filesystem::path in Platform.h

Since C++17 had introduced built-in standard path library, it is unnecessary to have custom path manipulation code to FoundationDB.

Start by reading Platform.h and locating the custom path-manipulation code described in the issue. Trace where that interface is used before assessing the migration scope. Done means the relevant path handling uses C++17's std::filesystem::path without leaving the old custom path logic in the affected area.

Written by the indexing model from the issue text.