RFR: 8359493: Refactor how aggregated mandatory warnings are handled in the compiler

[Archie Cobbs]

The compiler's handling of the aggregation of mandatory warnings into "notes" at the end of compilation can be refactored to simplify the code.

The `JCDiagnostic` class supports flags that alter how warnings are handled, e.g., `MANDATORY`, `NON_DEFERRABLE`, etc. So instead of having to log aggregated mandatory warnings through a separate channel (the `MandatoryWarningHandler`), these warnings could instead be logged just like any other warning, but with an `AGGREGATED` flag added. The actual aggregation can then be handled "behind the scenes" by the logging subsystem.

This will also make it easier to implement `@SuppressAnnotations` support for parser/tokenizer warnings which require aggregated mandatory warning notes such as warnings for preview features.

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Commit messages:
 - Refactor MandatoryWarningHandler to just be an aggregator.

Changes: https://git.openjdk.org/jdk/pull/25810/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25810&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8359493
  Stats: 834 lines in 10 files changed: 378 ins; 428 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/25810.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25810/head:pull/25810

PR: https://git.openjdk.org/jdk/pull/25810