Enable JVM IR for stdlib/reflect/test libraries

Changes in Gradle integration tests are needed because:
- in new-mpp-android, kotlin-stdlib-jdk8 is used, and JVM IR generates
  JDK 8-specific bytecode (invokedynamic). D8 needs to be configured to
  desugar it with source/target versions set to 1.8, otherwise it
  reports an error.
- in AndroidExtensionsManyVariants and AndroidIcepickProject, D8 fails
  with assertions enabled if AGP < 4.0.0 is used because of
  https://issuetracker.google.com/issues/148661132. The tests which use
  old AGP versions are probably not relevant anymore anyway.

Changes in kotlin-stdlib-runtime-merged.txt are caused by a slightly
different generation scheme of collection subclasses in JVM IR, and are
harmless.

(Previous attempt was at 15e978dbd311c2ba78ec32b394c21acde9811ccb.)

libraries/commonConfiguration.gradle

@@ -64,17 +64,6 @@ ext.configurePluginMarkers = { Project project, withEmptyJars = true ->
     PluginMarkersKt.publishPluginMarkers(project, withEmptyJars)
 }
 
-ext.configureJvmIrBackend = { Project project ->
-    project.tasks.withType(KotlinCompile.class) { task ->
-        task.kotlinOptions {
-            if (!project.kotlinBuildProperties.useIRForLibraries) {
-                useIR = false
-                freeCompilerArgs += "-Xuse-old-backend"
-            }
-        }
-    }
-}
-
 allprojects { project ->
     project.ext.configureSourcesJar = { lambda = {} ->
         ArtifactsKt.sourcesJar(project) { task ->