RFR: 8272884: Make VoidClosure::do_void pure virtual

[Albert Mingkun Yang]

Simple change of making VoidClosure::do_void pure virtual.

Test: build

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Commit messages:
 - voidclosure

Changes: https://git.openjdk.java.net/jdk/pull/5237/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5237&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272884
  Stats: 7 lines in 2 files changed: 0 ins; 6 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5237.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5237/head:pull/5237

PR: https://git.openjdk.java.net/jdk/pull/5237