RFR: 8266822: Rename MetaspaceShared::is_old_class to has_old_class_version

[Calvin Cheung]

Please review this simple patch for renaming the function from `MetaspaceShared::is_old_class` to `MetaspaceShared::has_old_class_version`. Also added some comment to the function.

Tests:
- [x] tier1, 2

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Commit messages:
 - 8266822: Rename MetaspaceShared::is_old_class to has_old_class_version

Changes: https://git.openjdk.java.net/jdk/pull/3983/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3983&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266822
  Stats: 18 lines in 6 files changed: 4 ins; 0 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3983.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3983/head:pull/3983

PR: https://git.openjdk.java.net/jdk/pull/3983