[clang][ExtractAPI] Refactor serializer to the CRTP

Refactor SerializerBase and SymbolGraphSerializer to use a visitor pattern described by the CRTP.

Reviewed By: dang

Differential Revision: https://reviews.llvm.org/D151477

GitOrigin-RevId: 06ff9770477d8c7378047b0171db4b25eba5d8dd
5 files changed