[jdk24] RFR: 8347646: module-info classfile missing the preview flag

[Jan Lahoda]

Hi all,

This pull request contains a backport of commit [bb93f67e](https://github.com/openjdk/jdk/commit/bb93f67ea8955216e81d1aef58d0ec8bf1fc9bb1) from the [openjdk/jdk](https://git.openjdk.org/jdk) repository.

The commit being backported was authored by Jan Lahoda on 14 Jan 2025 and was reviewed by Adam Sotona.

Thanks!

Original description:

Consider module-info.java like this:

module m {
    requires transitive java.base;
}


When compiled with `--enable-preview --source 24/25`, the resulting classfile is missing the preview flag.

This PR proposes to use `Preview.checkSourceLevel` to check the source level, which marks the source to use the preview feature.

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

Commit messages:
 - Backport bb93f67ea8955216e81d1aef58d0ec8bf1fc9bb1

Changes: https://git.openjdk.org/jdk/pull/23101/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23101&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347646
  Stats: 99 lines in 4 files changed: 81 ins; 9 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/23101.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23101/head:pull/23101

PR: https://git.openjdk.org/jdk/pull/23101