RFR: Support preview features in libraries

[Dan Smith]

Always allow preview class files, compile standard libs with --enable-preview

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Commit messages:
 - Always allow preview class files, compile standard libs with --enable-preview

Changes: https://git.openjdk.org/valhalla/pull/2037/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2037&range=00
  Stats: 17 lines in 7 files changed: 5 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/valhalla/pull/2037.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2037/head:pull/2037

PR: https://git.openjdk.org/valhalla/pull/2037