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

[Marcus Hirt]

On Thu, 6 May 2021 15:59:18 GMT, Henrik Dafgård <hdafgard at openjdk.org> wrote:

>> 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

Marked as reviewed by hirt (Lead).

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