RFR: 8328671: Mark `allocate_new_tlab` and `unsafe_max_tlab_alloc` of `CollectedHeap` as `pure virtual`

[Guoxiong Li]

Hi all,

This patch marks the methods `CollectedHeap::allocate_new_tlab` and `CollectedHeap::unsafe_max_tlab_alloc` as `pure virtual`. Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - JDK-8328671

Changes: https://git.openjdk.org/jdk/pull/18422/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18422&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328671
  Stats: 12 lines in 2 files changed: 0 ins; 10 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/18422.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18422/head:pull/18422

PR: https://git.openjdk.org/jdk/pull/18422