Change CodeGenSchedule 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

- Extract code for sorting and checking duplicate Records into a helper
  function and update `collectProcModels` to use the helper.
- Update `FeatureKeyValues` to:
  (a) Remove code for duplicate checks and use the helper.
  (b) Trim features with empty name explicitly to be able to use
      the helper.
- Make the sorting deterministic by using record name as a secondary
  key for sorting, and re-enable SubtargetFeatureUniqueNames.td test
  that was disabled due to the non-determinism of the error messages.
- Change wording of error message when duplicate records are found to
  be source code position agnostic, since `First` may not be before
  `Second` lexically.