[clang][extract-api] Add enum support

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