Some code cleanup before the big CL to use LabelManager to reduce peak
memory by 25%:
- Remove LabelManagerImpl and simply use LabelManager.
- Extract RvaVisitor to a separate class; adding specializations:
  - VectorRvaVisitor<T>: To prepare for for RVA and TypedRVA usage.
  - TrivialRvaVisitor: For unittests.

Review URL: https://codereview.chromium.org/1853943002

Cr-Commit-Position: refs/heads/master@{#385645}