exclude compiler.xml and artifacts when using Gradle (#3236)

In our IntelliJ projects where we are using Gradle, the file .idea/compiler.xml and files inside the .idea/artifacts folder are automatically generated by IntelliJ based on the Gradle build model. As these files are generated, they should be ignored in version control.

Global/JetBrains.gitignore

@@ -28,6 +28,8 @@
 # When using Gradle or Maven with auto-import, you should exclude module files,
 # since they will be recreated, and may cause churn.  Uncomment if using
 # auto-import.
+# .idea/artifacts
+# .idea/compiler.xml
 # .idea/modules.xml
 # .idea/*.iml
 # .idea/modules