RFR: CODETOOLS-7903008: JMH: Support incremental annotation processing for Gradle Java plugin

[Taylor Wicksell]

On Thu, 10 Jun 2021 17:53:42 GMT, Taylor Wicksell <github.com+301239+twicksell at openjdk.org> wrote:

> This change adds support for Gradle's [incremental annotation processing](https://docs.gradle.org/current/userguide/java_plugin.html#sec:incremental_annotation_processing). Without this support, developers using the JMH annotation processor in their Gradle projects will see their compilation time increased due to Gradle invalidating its incremental compilation cache.

Please hold off on merging. After running through some more complex benchmark examples, I've found a place where we are not using Filer to write classes. `StateObjectHandler.writeStateOverrides` seems problematic. Looking to see if I can fix that.

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PR: https://git.openjdk.java.net/jmh/pull/43