RFR: 8254557: Compiler crashes with java.lang.AssertionError: isSubtype UNKNOWN

[Guoxiong Li]

Hi all,

[JDK-8254557](https://bugs.openjdk.java.net/browse/JDK-8254557) is similar to
[JDK-8203277](https://bugs.openjdk.java.net/browse/JDK-8203277). The method `Attr.preFlow` shouldn't visit  class
definitions and lambda expressions that have not yet been entered and attributed.

The  [JDK-8181287](https://bugs.openjdk.java.net/browse/JDK-8231826) submitted some code that uses `Attr.preFlow`
which  causes this bug.
Thank you for taking the time to review.

Best Regards.

-------------

Commit messages:
 - Polish some comments.
 - 8254557: Compiler crashes with java.lang.AssertionError: isSubtype UNKNOWN

Changes: https://git.openjdk.java.net/jdk/pull/718/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=718&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254557
  Stats: 145 lines in 2 files changed: 145 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/718.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/718/head:pull/718

PR: https://git.openjdk.java.net/jdk/pull/718