Rename -Xno-use-ir -> -Xuse-old-backend, add Gradle option

As soon as JVM IR is enabled by default (in language version 1.5), use
the CLI argument `-Xuse-old-backend` or Gradle option `useOldBackend` to
switch to the old JVM backend.

libraries/tools/kotlin-gradle-plugin-api/src/main/kotlin/org/jetbrains/kotlin/gradle/dsl/KotlinCommonOptions.kt

@@ -17,4 +17,4 @@ interface KotlinCommonOptions  : org.jetbrains.kotlin.gradle.dsl.KotlinCommonToo
      * Default value: null
      */
      var languageVersion: kotlin.String?
-}
+}