RFR: 8325053: Serial: Move Generation::save_used_region to TenuredGeneration [v2]

[Albert Mingkun Yang]

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

Albert Mingkun Yang has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  s1-save-used-region

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17654/files
  - new: https://git.openjdk.org/jdk/pull/17654/files/d7de9ec7..ee5e0d6b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17654&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17654&range=00-01

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 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