RFR: 7645: Add isHidden() method to IMCMethod interface

[Henrik Dafgård]

Simply add a method to allow access to the already parsed flag determining whether or not a method is marked as hidden
or not.

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Commit messages:
 - Add isHidden method to IMCMethod interface

Changes: https://git.openjdk.java.net/jmc/pull/70/files
 Webrev: https://webrevs.openjdk.java.net/jmc/70/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7645
  Stats: 23 lines in 4 files changed: 23 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jmc/pull/70.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/70/head:pull/70

PR: https://git.openjdk.java.net/jmc/pull/70