RFR: 8325053: Serial: Move Generation::save_used_region to TenuredGeneration [v3]

[Albert Mingkun Yang]

> Trivial moving a field and its accessors from superclass to subclass.

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17654/files
  - new: https://git.openjdk.org/jdk/pull/17654/files/ee5e0d6b..421b44b3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17654&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17654&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17654.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17654/head:pull/17654

PR: https://git.openjdk.org/jdk/pull/17654