Migrate LLDB TableGen backend to use const RecordKeeper. This is a part of effort to have better const correctness in TableGen backends: https://discourse.llvm.org/t/psa-planned-changes-to-tablegen-getallderiveddefinitions-api-potential-downstream-breakages/81089
Instead of polluting the LLDBTableGenBackends header with helpers used by both emitters, move them into a separate files. llvm-svn: 367377