RFR: 8353757: Log class should have a proper clear() method

[Archie Cobbs]

This is a simple refactoring to add a method `Log.clear()`, move `ReusableLog.clear()`'s clearing operations into it and invoke `super.clear()` instead, and add two more fields that should be included in the clear operation but were not because they were added to `Log` after `ReusableLog.clear()` was added and were missed.

A natural question: Are there any other fields in `Log` that should be cleared in `Log.clear()`? The two I've added (`nsuppressederrors` and `nsuppressedwarns`) are the only two that weren't there when `ReusableLog` as added, so presumably not, but it's possible.

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Commit messages:
 - Add new method Log.clear().

Changes: https://git.openjdk.org/jdk/pull/24460/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24460&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353757
  Stats: 19 lines in 2 files changed: 12 ins; 2 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/24460.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24460/head:pull/24460

PR: https://git.openjdk.org/jdk/pull/24460