RFR: Support preview features in libraries [v2]

[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:

  change Referencies record to use static method

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/2037/files
  - new: https://git.openjdk.org/valhalla/pull/2037/files/fc719e7d..dad14789

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=2037&range=01
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=2037&range=00-01

  Stats: 6 lines in 1 file changed: 0 ins; 2 del; 4 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