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

[Archie Cobbs]

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.

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Commit messages:
 - Add missing warn() pass to sequential compiler steps.

Changes: https://git.openjdk.org/jdk/pull/23223/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23223&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348212
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/23223.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23223/head:pull/23223

PR: https://git.openjdk.org/jdk/pull/23223