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)
 

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/jvmArguments.kt

@@ -291,6 +291,8 @@ fun CompilerConfiguration.configureAdvancedJvmOptions(arguments: K2JVMCompilerAr
     }
 
     arguments.declarationsOutputPath?.let { put(JVMConfigurationKeys.DECLARATIONS_JSON_PATH, it) }
+
+    put(CommonConfigurationKeys.RUN_LOWERINGS_IN_PARALLEL, arguments.runLoweringsInParallel)
 }
 
 fun CompilerConfiguration.configureKlibPaths(arguments: K2JVMCompilerArguments) {