[clang][extract-api] Add struct support

- Add `StructFieldRecord` and `StructRecord` to store API information
  for structs
- Implement `VisitRecordDecl` in `ExtractAPIVisitor`
- Implement Symbol Graph serialization for struct records.
- Add test case for struct records.

Depends on D121873

Differential Revision: https://reviews.llvm.org/D122202