RFR: 8344148: Add an explicit compiler phase for warning generation

[Archie Cobbs]

Please review which does some minor refactoring to the compiler:
* Create a new `WARN` phase which can be a dedicated home for (new) lint/warning logic
* Create a new `WarningAnalyzer` singleton whose job is to invoke such lint/warning logic
* Move `ThisEscapeAnalyzer` out of `Flow` (where it doesn't belong) and into `WarningAnalyzer`
* Refactor `ThisEscapeAnalyzer` to be a context singleton like all other such classes

See [JDK-8344148](https://bugs.openjdk.org/browse/JDK-8344148) for details.

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Commit messages:
 - Update copyright years.
 - Add an explicit compiler phase for warning generation.

Changes: https://git.openjdk.org/jdk/pull/22088/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22088&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344148
  Stats: 218 lines in 19 files changed: 190 ins; 2 del; 26 mod
  Patch: https://git.openjdk.org/jdk/pull/22088.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22088/head:pull/22088

PR: https://git.openjdk.org/jdk/pull/22088