RFR: 8350029: Illegal invokespecial interface not caught by verification

[David Holmes]

As described in the JBS problem statement there was a missing verification check for bad `invokespecial` call, though it did result in a runtime exception. That gap has now been closed by checking if the target of the invocation is an interface type, which we can check when the type gets loaded as part of the assignability check.

The existing code was very difficult to follow so I have added significant commentary in the verifier. I also improved the naming in the assignability checking code to make it easier to follow. Finally, the existing linkage error was using an incorrect error message to describe the problem it was encountering.

Testing:
 - new test case
 - tiers 1-3

Thanks

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Commit messages:
 - 8350029: Illegal invokespecial interface not caught by verification

Changes: https://git.openjdk.org/jdk/pull/25538/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25538&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350029
  Stats: 263 lines in 8 files changed: 209 ins; 1 del; 53 mod
  Patch: https://git.openjdk.org/jdk/pull/25538.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25538/head:pull/25538

PR: https://git.openjdk.org/jdk/pull/25538