Discovered while writing a test for #16513 that `ClassSignature.superclassSignature()` does not return a `ClassTypeSig`, yet [JVM Spec](https://docs.oracle.com/javase/specs/jvms/se21/html/jvms-4.html#jvms-4.7.9.1-4100) requires it to be one. This patch adds such a requirement to the accessors, factories, and the parsing logic.
------------- Commit messages: - 8319463: ClassSignature should have superclass and superinterfaces as ClassTypeSig Changes: https://git.openjdk.org/jdk/pull/16514/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16514&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8319463 Stats: 22 lines in 3 files changed: 8 ins; 0 del; 14 mod Patch: https://git.openjdk.org/jdk/pull/16514.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/16514/head:pull/16514 PR: https://git.openjdk.org/jdk/pull/16514
