JVM_IR: perform file lowerings in parallel

Selected by -Xir-run-lowerings-in-paralled compiler flag.

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2JVMCompilerArguments.kt

@@ -114,6 +114,12 @@ class K2JVMCompilerArguments : CommonCompilerArguments() {
     )
     var doNotClearBindingContext: Boolean by FreezableVar(false)
 
+    @Argument(
+        value = "-Xir-run-lowerings-in-parallel",
+        description = "When using the IR backend, run lowerings for each file in parallel"
+    )
+    var runLoweringsInParallel: Boolean by FreezableVar(false)
+
     @Argument(value = "-Xmodule-path", valueDescription = "<path>", description = "Paths where to find Java 9+ modules")
     var javaModulePath: String? by NullableStringFreezableVar(null)