RFR: 8299437: Make InetSocketAddressHolder shallowly immutable

[Per Minborg]

This PR proposes to make the class `java.net.InetSocketAddress.InetSocketAddressHolder` shallowly immutable

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Commit messages:
 - Make InetSocketAddressHolder shallowly immutable

Changes: https://git.openjdk.org/jdk/pull/11809/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11809&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299437
  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/11809.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11809/head:pull/11809

PR: https://git.openjdk.org/jdk/pull/11809