RFR: 8330006: Serial: Extract out ContiguousSpace::block_start_const

[Guoxiong Li]

Hi all,

This patch moves the method `ContiguousSpace::block_start_const` to `DefNewGeneration` (note: not the parent class `Generation`) and moves the method `TenuredSpace::block_start_const` to `TenuredGeneration`. These two methods don't need  the same signature so I don't add a virtual method to the parent class `Generation`.

Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - JDK-8330006

Changes: https://git.openjdk.org/jdk/pull/18722/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18722&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330006
  Stats: 99 lines in 8 files changed: 43 ins; 43 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/18722.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18722/head:pull/18722

PR: https://git.openjdk.org/jdk/pull/18722