RFR: Support preview features in libraries [v3]

[Dan Smith]

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

Dan Smith has updated the pull request incrementally with one additional commit since the last revision:

  langtools test fixes

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/2037/files
  - new: https://git.openjdk.org/valhalla/pull/2037/files/dad14789..1a094540

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=2037&range=02
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=2037&range=01-02

  Stats: 9 lines in 3 files changed: 5 ins; 2 del; 2 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