RFR: JDK-8277520: Implement JDK-8 default methods for IdentityHashMap

[liach]

Might need a CSR as now `computeIfAbsent` `computeIfPresent` `compute` `merge` would throw CME if the functions modified the map itself, and there are corresponding specification changes.

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Commit messages:
 - Also test cme for identity hash map
 - Fix putIfAbsent
 - JDK-8277520: Implement JDK-8 default methods for IdentityHashMap

Changes: https://git.openjdk.java.net/jdk/pull/6532/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6532&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277520
  Stats: 415 lines in 2 files changed: 313 ins; 83 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6532.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6532/head:pull/6532

PR: https://git.openjdk.java.net/jdk/pull/6532