RFR: 8353757: Log class should have a proper clear() method [v3]

[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.

Archie Cobbs has updated the pull request incrementally with two additional commits since the last revision:

 - Reset the DiagnosticHandler stack in clear().
 - Revert accidentally included unrelated changes.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/24460/files
  - new: https://git.openjdk.org/jdk/pull/24460/files/a1108ac7..a9d58450

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24460&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24460&range=01-02

  Stats: 13 lines in 2 files changed: 2 ins; 7 del; 4 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