Don't diff records / enums already diffed through referenced type diffs.

This causes unnecessary verbosity in diff reports.

Test: The following:
  Without this change:
  header-abi-diff -arch arm64 -lib libcpp_referenced_records -old \
  tests/reference_dumps/arm64/libgolden_cpp.so.lsdump -new \
  tests/reference_dumps/arm64/libgolden_cpp_change_member_access.so.lsdump -o \
  unref_diff -check-all-apis; unref_diff has LowVolumeSpeaker as an
  unreferenced diff as well as a referenced diff.

  With this change:
  Run the same test command as above; unref_diff has LowVolumeSpeaker
  only as a referenced diff.

Change-Id: Iddb2ed2f759637f53b00fd1398395a1b1c207116