Compiler: restore an opportunity to switch light tree mode OFF

#KT-55996 Fixed

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

@@ -46,7 +46,7 @@ fun <A : CommonCompilerArguments> CompilerConfiguration.setupCommonArguments(
 
     val usesK2 = arguments.useK2 || languageVersionSettings.languageVersion.usesK2
     put(CommonConfigurationKeys.USE_FIR, usesK2)
-    put(CommonConfigurationKeys.USE_LIGHT_TREE, usesK2)
+    put(CommonConfigurationKeys.USE_LIGHT_TREE, arguments.useFirLT)
 }
 
 fun <A : CommonCompilerArguments> CompilerConfiguration.setupLanguageVersionSettings(arguments: A) {

compiler/testData/cli/jvm/firLightTree.kt

@@ -0,0 +1,4 @@
+fun List<String>.test() {
+    this += "Alpha"
+    java.lang.System = null
+}

compiler/testData/cli/jvm/firLightTreeOff.args

@@ -0,0 +1,6 @@
+$TESTDATA_DIR$/firLightTree.kt
+-language-version
+2.0
+-Xuse-fir-lt=false
+-d
+$TEMP_DIR$

compiler/testData/cli/jvm/firLightTreeOff.out

@@ -0,0 +1,8 @@
+warning: language version 2.0 is experimental, there are no backwards compatibility guarantees for new language and library features
+compiler/testData/cli/jvm/firLightTree.kt:2:5: error: variable expected
+    this += "Alpha"
+    ^
+compiler/testData/cli/jvm/firLightTree.kt:3:15: error: classifier 'class System : Any' does not have a companion object, and thus must be initialized here
+    java.lang.System = null
+              ^
+COMPILATION_ERROR

compiler/testData/cli/jvm/firLightTreeOn.args

@@ -0,0 +1,6 @@
+$TESTDATA_DIR$/firLightTree.kt
+-language-version
+2.0
+-Xuse-fir-lt=true
+-d
+$TEMP_DIR$

compiler/testData/cli/jvm/firLightTreeOn.out

@@ -0,0 +1,5 @@
+warning: language version 2.0 is experimental, there are no backwards compatibility guarantees for new language and library features
+compiler/testData/cli/jvm/firLightTree.kt:3:5: error: classifier 'class System : Any' does not have a companion object, and thus must be initialized here
+    java.lang.System = null
+    ^
+COMPILATION_ERROR

compiler/tests-gen/org/jetbrains/kotlin/cli/CliTestGenerated.java

@@ -462,6 +462,16 @@ public class CliTestGenerated extends AbstractCliTest {
             runTest("compiler/testData/cli/jvm/firHello20WithOldLV.args");
         }
 
+        @TestMetadata("firLightTreeOff.args")
+        public void testFirLightTreeOff() throws Exception {
+            runTest("compiler/testData/cli/jvm/firLightTreeOff.args");
+        }
+
+        @TestMetadata("firLightTreeOn.args")
+        public void testFirLightTreeOn() throws Exception {
+            runTest("compiler/testData/cli/jvm/firLightTreeOn.args");
+        }
+
         @TestMetadata("firMultiplatformCompilationWithError.args")
         public void testFirMultiplatformCompilationWithError() throws Exception {
             runTest("compiler/testData/cli/jvm/firMultiplatformCompilationWithError.args");