K2: introduce LT mode config property

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

@@ -23,6 +23,7 @@ fun <A : CommonCompilerArguments> CompilerConfiguration.setupCommonArguments(
 ) {
     put(CommonConfigurationKeys.DISABLE_INLINE, arguments.noInline)
     put(CommonConfigurationKeys.USE_FIR, arguments.useK2)
+    put(CommonConfigurationKeys.USE_LIGHT_TREE, arguments.useFirLT)
     put(CommonConfigurationKeys.USE_FIR_EXTENDED_CHECKERS, arguments.useFirExtendedCheckers)
     put(CommonConfigurationKeys.EXPECT_ACTUAL_LINKER, arguments.expectActualLinker)
     putIfNotNull(CLIConfigurationKeys.INTELLIJ_PLUGIN_ROOT, arguments.intellijPluginRoot)

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/K2JVMCompiler.kt

@@ -142,7 +142,7 @@ class K2JVMCompiler : CLICompiler<K2JVMCompilerArguments>() {
             val targetDescription = chunk.map { input -> input.getModuleName() + "-" + input.getModuleType() }.let { names ->
                 names.singleOrNull() ?: names.joinToString()
             }
-            if (configuration.getBoolean(CommonConfigurationKeys.USE_FIR) && arguments.useFirLT /* TODO: consider storing in the configuration instead of using args here directly */) {
+            if (configuration.getBoolean(CommonConfigurationKeys.USE_FIR) && configuration.getBoolean(CommonConfigurationKeys.USE_LIGHT_TREE))  {
                 val projectEnvironment =
                     createProjectEnvironment(configuration, rootDisposable, EnvironmentConfigFiles.JVM_CONFIG_FILES, messageCollector)
 

compiler/config/src/org/jetbrains/kotlin/config/CommonConfigurationKeys.kt

@@ -53,6 +53,9 @@ object CommonConfigurationKeys {
     @JvmField
     val USE_FIR = CompilerConfigurationKey.create<Boolean>("front-end IR")
 
+    @JvmField
+    val USE_LIGHT_TREE = CompilerConfigurationKey.create<Boolean>("light tree")
+
     @JvmField
     val EXPECT_ACTUAL_LINKER = CompilerConfigurationKey.create<Boolean>("Experimental expect/actual linker")