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.

build.gradle.kts

@@ -193,6 +193,7 @@ if (!project.hasProperty("versions.kotlin-native")) {
 val intellijUltimateEnabled by extra(project.kotlinBuildProperties.intellijUltimateEnabled)
 val effectSystemEnabled by extra(project.getBooleanProperty("kotlin.compiler.effectSystemEnabled") ?: false)
 val newInferenceEnabled by extra(project.getBooleanProperty("kotlin.compiler.newInferenceEnabled") ?: false)
+val useJvmIrBackend by extra(project.getBooleanProperty("kotlin.build.useIR") ?: false)
 
 val intellijSeparateSdks = project.getBooleanProperty("intellijSeparateSdks") ?: false
 
@@ -426,6 +427,10 @@ allprojects {
     tasks.withType<org.jetbrains.kotlin.gradle.dsl.KotlinJvmCompile> {
         kotlinOptions {
             freeCompilerArgs = commonCompilerArgs + jvmCompilerArgs
+
+            if (useJvmIrBackend) {
+                useIR = true
+            }
         }
     }
 

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