[cfe] Refactor ClassMembersNodeBuilder.build

This splits the build method into smaller classes responsible for each
part of the building/validation process.

Change-Id: I05c5cbef91c75a168dc8e7ec6d91bf484a0a7451
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/329462
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

Author: johnniwinther