RFR: 8348212: Need to add warn() step to JavacTaskImpl after JDK-8344148 [v2]

[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.

Archie Cobbs has updated the pull request incrementally with one additional commit since the last revision:

  Add a regression test.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23223/files
  - new: https://git.openjdk.org/jdk/pull/23223/files/d07dc1a3..c7b6fcb5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23223&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23223&range=00-01

  Stats: 85 lines in 1 file changed: 85 ins; 0 del; 0 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