Compiler: restore an opportunity to switch light tree mode OFF

#KT-55996 Fixed

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