RFR: 8337876: [IR Framework] Add support for IR tests with @Stable

[Christian Hagedorn]

It is currently not possible to write IR tests with `@Stable` annotations because one need to somehow add the IR test classes to the boot classpath. This patch provides support to write such IR tests. I've added a section to the README to provide guidance on how this can be done.

This is motivated by https://github.com/openjdk/jdk/pull/19635.

I've tested this patch by taking the current patch of https://github.com/openjdk/jdk/pull/19635, dropping the `RestrictStable` flag and modifying the tests to work with the new IR framework feature:

TestFramework testFramework = new TestFramework();
testFramework
        .addFlags("-XX:-TieredCompilation",
                  "-XX:+UseParallelGC")
        .addTestClassesToBootClassPath()
        .start();


Thanks,
Christian

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Commit messages:
 - 8337876: [IR Framework] Add support for IR tests with @Stable

Changes: https://git.openjdk.org/jdk/pull/20477/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20477&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337876
  Stats: 36 lines in 4 files changed: 26 ins; 1 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/20477.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20477/head:pull/20477

PR: https://git.openjdk.org/jdk/pull/20477