RFR: 8348212: Need to add warn() step to JavacTaskImpl after JDK-8344148 [v3]

[Maurizio Cimadamore]

On Fri, 24 Jan 2025 17:24:08 GMT, Archie Cobbs <acobbs at openjdk.org> wrote:

>> In [JDK-8344148](https://bugs.openjdk.org/browse/JDK-8344148) a new `warn()` compiler phase was added and `JavaCompiler.java` was updated accordingly.
>> 
>> However, the class `JavacTaskImpl` also walks through the compiler phases step-by-step, but the new `warn()` step was never added there. This will cause some warnings to not be emitted when that API is used.
>> 
>> This PR adds the missing `warn()` calls.
>
> Archie Cobbs has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Avoid having the regression test class extend another regression test class.

Marked as reviewed by mcimadamore (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/23223#pullrequestreview-2573286665