KT-52217 Rename 'use-fir' to 'use-k2', update message

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

@@ -22,7 +22,7 @@ fun <A : CommonCompilerArguments> CompilerConfiguration.setupCommonArguments(
     createMetadataVersion: ((IntArray) -> BinaryVersion)? = null
 ) {
     put(CommonConfigurationKeys.DISABLE_INLINE, arguments.noInline)
-    put(CommonConfigurationKeys.USE_FIR, arguments.useFir)
+    put(CommonConfigurationKeys.USE_FIR, arguments.useK2)
     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/compiler/FirKotlinToJvmBytecodeCompiler.kt

@@ -82,7 +82,7 @@ object FirKotlinToJvmBytecodeCompiler {
 
         messageCollector.report(
             STRONG_WARNING,
-            "ATTENTION!\n This build uses in-dev FIR: \n  -Xuse-fir"
+            "ATTENTION!\n This build uses experimental K2 compiler: \n  -Xuse-k2"
         )
 
         projectConfiguration.get(ComponentRegistrar.PLUGIN_COMPONENT_REGISTRARS)?.let { pluginComponentRegistrars ->

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/compiler/pipeline/compilerPipeline.kt

@@ -97,7 +97,7 @@ fun compileModulesUsingFrontendIrAndLightTree(
 
     messageCollector.report(
         CompilerMessageSeverity.STRONG_WARNING,
-        "ATTENTION!\n This build uses in-dev FIR: \n  -Xuse-fir"
+        "ATTENTION!\n This build uses experimental K2 compiler: \n  -Xuse-k2"
     )
 
     val outputs = mutableListOf<GenerationState>()

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

@@ -230,7 +230,7 @@ fun CompilerConfiguration.configureAdvancedJvmOptions(arguments: K2JVMCompilerAr
 
     // TODO: ignore previous configuration value when we do not need old backend in scripting by default
     val useOldBackend = arguments.useOldBackend || (!arguments.useIR && get(JVMConfigurationKeys.IR) == false)
-    val useIR = arguments.useFir ||
+    val useIR = arguments.useK2 ||
             if (languageVersionSettings.supportsFeature(LanguageFeature.JvmIrEnabledByDefault)) {
                 !useOldBackend
             } else {