RFR: 8344148: Add an explicit compiler phase for warning generation [v2]

[Archie Cobbs]

> Please review this patch 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.

Archie Cobbs has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Merge branch 'master' into JDK-8344148
 - Merge branch 'master' into JDK-8344148
 - Update copyright years.
 - Add an explicit compiler phase for warning generation.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/22088/files
  - new: https://git.openjdk.org/jdk/pull/22088/files/d48c552d..b1ca46d9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22088&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22088&range=00-01

  Stats: 244382 lines in 5571 files changed: 148630 ins; 73957 del; 21795 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