Warn if the derived class and the base class have the same AddSetting call instead of silently ignoring it.
Enabled by rP22958 (refs rP22952 / D2314, rP22969 / D2318).
Helps structuring the GUI so that AddSetting and GetSetting are in the same file, which defragments code and may allow for future optimizations (setting values as class members for Draw calls), refs D2313.
Differential Revision: https://code.wildfiregames.com/D2320
Tested on: clang 8.0.1, Jenkins