RFR: 8338420: GenShen: Forward declare card table for Shenandoah barrier set

[William Kemper]

Forward declare card table, remove completed TODO comment

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Commit messages:
 - Forward declare card table, remove completed TODO comment

Changes: https://git.openjdk.org/shenandoah/pull/470/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=470&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338420
  Stats: 12 lines in 6 files changed: 8 ins; 4 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/470.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/470/head:pull/470

PR: https://git.openjdk.org/shenandoah/pull/470