Add property kotlin.build.useIR to enable JVM IR in Kotlin build

Right now this is only supposed to be turned on locally (`./gradlew
-Pkotlin.build.useIR=true ...`), to be able to quickly try how JVM
IR-compiled Kotlin compiler works.

gradle.properties

@@ -5,6 +5,7 @@ defaultSnapshotVersion=1.4.255-SNAPSHOT
 
 kotlin.compiler.effectSystemEnabled=true
 kotlin.compiler.newInferenceEnabled=true
+#kotlin.build.useIR=true
 #maven.repository.mirror=http://repository.jetbrains.com/remote-repos/
 #bootstrap.kotlin.repo=https://dl.bintray.com/kotlin/kotlin-dev
 #bootstrap.kotlin.version=1.1.50-dev-1451