Use an ID-to-name map when volume has no company string

This is written so that the result of GetCompanyFromID never is cached
(except on Android?). Caching is unnecessary because the string can be
obtained quickly at runtime, and not caching it means that the cache
doesn't have to be invalidated when GetCompanyFromID is edited.

Source/Core/DiscIO/Volume.h

@@ -125,5 +125,6 @@ protected:
 // Generic Switch function for all volumes
 IVolume::ECountry CountrySwitch(u8 country_code);
 u8 GetSysMenuRegion(u16 _TitleVersion);
+std::string GetCompanyFromID(const std::string& company_id);
 
 } // namespace