RFR: 8293348: A false cyclic inheritance error reported

[Jan Lahoda]

Consider these two classes:

non-sealed interface T2 extends T {}
sealed interface T permits T2 {}


With a bit non-standard set-up, it may happen that:
- as part of `TypeEnter.HierarchyPhase`, `Check.checkNonCyclicInternal` is called for `T2`,
- that will mark T2 as LOCKED, and call `checkNonCyclicInternal` for its supertype `T`, which is uncompleted,
- `T` will be completed, so `TypeEnter.HierarchyPhase` will be called for it,
- `TypeEnter.HierarchyPhase` for `T` will try to attribute the permitted types of `T`,
- that will invoke `Check.checkNonCyclicInternal` for `T2` again,
- but the class is already locked, and hence a cycle error is produced.

The proposed solution is to defer permitted type handling to a later point in time, specifically to the `HeaderPhase`. The `HierarchyPhase` should only handle the type hierarchy (i.e. the supertypes of a given type), and should not handle cycles. Hence the `HeaderPhase` seems like a better place to handle the permitted types, similarly to type variable bounds handling.

Setting `isPermittedExplicit` is moved to `Enter` - seems the value of the flag can be inferred easily from the AST node, and can simply be used later.

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Commit messages:
 - 8293348: A false cyclic inheritance error reported

Changes: https://git.openjdk.org/jdk/pull/10171/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10171&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293348
  Stats: 110 lines in 3 files changed: 64 ins; 44 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/10171.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10171/head:pull/10171

PR: https://git.openjdk.org/jdk/pull/10171