[lworld] RFR: LoadableDescriptorsAttrTest2 uses preview API Attributes.loadableDescriptors

[Chen Liang]

Fixes new javac failures caused by #1542 when Attributes.loadableDescriptors is now preview. (It should probably be a reflective preview API; that will be addressed in #1439)

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Commit messages:
 - LoadableDescriptorsAttrTest2 uses preview API Attributes.loadableDescriptors

Changes: https://git.openjdk.org/valhalla/pull/1543/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1543&range=00
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/1543.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1543/head:pull/1543

PR: https://git.openjdk.org/valhalla/pull/1543