Add support for enum records
- Add `EnumConstantRecord` and `EnumRecord` to store API information for
enums
- Implement `VisitEnumDecl` in `ExtractAPIVisitor`
- Implement serializatin for enum records and `MemberOf` relationship
- Add test case for enum records
- Few other improvements
Depends on D122160
Differential Revision: https://reviews.llvm.org/D121873
Zixu Wang