RFR: 8063: IMCFrame Type cache not synchronized

[Pierre-Yves Bigourdan]

The type cache used in the IMCFrame Type inner class isn't synchronized and can thus cause a concurrent modification exception during e.g. JFR parsing.

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Commit messages:
 - 8063: IMCFrame Type cache not synchronized

Changes: https://git.openjdk.org/jmc/pull/476/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=476&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8063
  Stats: 10 lines in 1 file changed: 2 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jmc/pull/476.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/476/head:pull/476

PR: https://git.openjdk.org/jmc/pull/476