[clang][ExtractAPI] Refactor C++ method and field visitation

Refactor visitation for C++ record children by following the Visitor's CRTP.
Expand VisitCXXField, VisitCXXMethod for non-templates and introduce VisitCXXConstructor, VisitCXXDestructor.
Handle relationships by finding the parent's Record via USR from DeclContext.

Depends on D158029

Reviewed By: dang

Differential Revision: https://reviews.llvm.org/D158031