[jdk11u-dev] RFR: 8313816: Accessing jmethodID might lead to spurious crashes

[Jaroslav Bachorik]

This backport is, unfortunately, not a clean one.

The functional changes applied almost cleanly, modulo the change of 'nullptr' to 'NULL' in the source code.

The problematic part is the testing - the test had to be adjusted for JDK 11, I had to pull in two test support files (or could have extracted the support methods but it felt like having the full files would be easier to review considering differences against the original) and I had to change the whitebox function to trigger purging of the previous class version.

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Commit messages:
 - Adjust for metaspace state
 - Use NULL instead of nullptr
 - Add test support file
 - Resolve merge conflicts
 - 8313816: Accessing jmethodID might lead to spurious crashes

Changes: https://git.openjdk.org/jdk11u-dev/pull/2321/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2321&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313816
  Stats: 1226 lines in 11 files changed: 1224 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2321.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2321/head:pull/2321

PR: https://git.openjdk.org/jdk11u-dev/pull/2321