RFR: JDK-8302129: Make MetaspaceReclaimPolicy a diagnostic switch

[Thomas Stuefe]

See associated CSR for background and motivation: https://bugs.openjdk.org/browse/JDK-8302130 .

This patch is technically trivial. CSR is approved.

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Commit messages:
 - JDK-8302129-make-Metaspace-reclaim-policy-diagnostic

Changes: https://git.openjdk.org/jdk/pull/12540/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12540&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302129
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12540.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12540/head:pull/12540

PR: https://git.openjdk.org/jdk/pull/12540