RFR: 7164: Add support for hidden classes [v3]

[Henrik Dafgård]

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

Henrik Dafgård has updated the pull request incrementally with one additional commit since the last revision:

  Update IMCType.java

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Changes:
  - all: https://git.openjdk.java.net/jmc/pull/247/files
  - new: https://git.openjdk.java.net/jmc/pull/247/files/60316e62..804d504d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jmc&pr=247&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jmc&pr=247&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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