[sealed-types] RFR: Attempting to fix separate compilation with sealed classes.

[Jan Lahoda]

Basically, adjusting the sealed supertype compilation to not depend on TypeEnter.

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Commit messages:
 - Attempting to fix separate compilation with sealed classes.

Changes: https://git.openjdk.java.net/amber/pull/19/files
 Webrev: https://webrevs.openjdk.java.net/amber/19/webrev.00
  Stats: 218 lines in 4 files changed: 83 ins; 116 del; 19 mod
  Patch: https://git.openjdk.java.net/amber/pull/19.diff
  Fetch: git fetch https://git.openjdk.java.net/amber pull/19/head:pull/19

PR: https://git.openjdk.java.net/amber/pull/19