RFR: 7164: Add support for hidden classes

[Henrik Dafgård]

Adds support for JDK 15 hidden classes to the JFR parser and IMCType interface (defaults to `false`).

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Commit messages:
 - Add support for hidden classes

Changes: https://git.openjdk.java.net/jmc/pull/247/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=247&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7164
  Stats: 15 lines in 2 files changed: 15 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jmc/pull/247.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/247/head:pull/247

PR: https://git.openjdk.java.net/jmc/pull/247