[jdk11u-dev] RFR: 8268364: jmethod clearing should be done during unloading

[Kerem Kat]

Backport of [8268364](https://bugs.openjdk.org/browse/JDK-8268364), single hunk conflict resolved by accepting "empty" from the backport commit vs. `clear_jmethod_ids` invocation in the dtor. Backport commit moves the `clear_jmethod_ids` invocation to `unload` method instead.

It fixes crashes when accessing jmethodIDs of a class being unloaded.

Here is the reproducer that crashes JVM in ~1 second without the patch, but works fine with it: [gist](https://gist.github.com/krk/a0800bac5bc5a01709be85637285a965).

This fix also resolves the issue reported at https://github.com/async-profiler/async-profiler/issues/974 for Java 11.

To run the repro:


javac Main.java
gcc -shared -I"$JAVA_HOME/include" -I"$JAVA_HOME/include/linux" -fPIC repro.cpp -orepro.so

java -agentpath:"$(pwd)/repro.so" -Xmx100m -XX:+UseConcMarkSweepGC -XX:+CMSClassUnloadingEnabled -XX:+ExplicitGCInvokesConcurrent Main

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Commit messages:
 - Backport 3d84398d128bb2eed6280ebbc3f57afb3b89908f

Changes: https://git.openjdk.org/jdk11u-dev/pull/2935/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2935&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8268364
  Stats: 31 lines in 2 files changed: 20 ins; 9 del; 2 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2935.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2935/head:pull/2935

PR: https://git.openjdk.org/jdk11u-dev/pull/2935