RFR: JDK-8309568 javac crashes attempting to -Xprint on a class file of an unnamed class

[Jim Laskey]

Test development for [JDK-8309503](https://bugs.openjdk.org/browse/JDK-8309503) found that javac crashed attempting to -Xprint on a class file of an unnamed class. The test for ClassReader detecting of unnamed class was forcing completion of the owner class. The new code uses a flags only test.

Related test is with https://bugs.openjdk.org/browse/JDK-8309503 .

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Commit messages:
 - Better check for top level class (non-completing)

Changes: https://git.openjdk.org/jdk/pull/14344/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14344&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309568
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14344.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14344/head:pull/14344

PR: https://git.openjdk.org/jdk/pull/14344