[Clang Importer] Refactor information pertaining to enums out
Introduces new class, EnumInfo, just for the implementation of the
ClangImporter to encapsulate various computed information about the
enum we're importing.
This helps refactor some functionality, aids clarity, and also
prevents us from repeating calculations multiple times, as we were
doing with classifyEnum's macro-expansion tracking. Provides a base
where we can add more heavy lifting in classifyEnum in the future.
