Minor: refactor extended compiler checks

Use an analysis flag instead of a stub language feature

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

@@ -382,6 +382,7 @@ abstract class CommonCompilerArguments : CommonToolArguments() {
                 CompilerMessageSeverity.ERROR,
                 "Unknown value for parameter -Xexplicit-api: '$explicitApi'. Value should be one of ${ExplicitApiMode.availableValues()}"
             )
+            put(AnalysisFlags.extendedCompilerChecks, extendedCompilerChecks)
         }
     }
 
@@ -431,10 +432,6 @@ abstract class CommonCompilerArguments : CommonToolArguments() {
                 put(LanguageFeature.InferenceCompatibility, LanguageFeature.State.ENABLED)
             }
 
-            if (extendedCompilerChecks) {
-                put(LanguageFeature.ExtendedCompilerChecks, LanguageFeature.State.ENABLED)
-            }
-
             if (progressiveMode) {
                 LanguageFeature.values().filter { it.kind.enabledInProgressiveMode }.forEach {
                     // Don't overwrite other settings: users may want to turn off some particular

compiler/cli/src/org/jetbrains/kotlin/cli/common/messages/AnalyzerWithCompilerReport.kt

@@ -25,8 +25,8 @@ import org.jetbrains.kotlin.analyzer.AnalysisResult
 import org.jetbrains.kotlin.cli.common.CLIConfigurationKeys
 import org.jetbrains.kotlin.cli.common.messages.CompilerMessageSeverity.*
 import org.jetbrains.kotlin.codegen.state.IncompatibleClassTrackerImpl
+import org.jetbrains.kotlin.config.AnalysisFlags
 import org.jetbrains.kotlin.config.CompilerConfiguration
-import org.jetbrains.kotlin.config.LanguageFeature
 import org.jetbrains.kotlin.config.LanguageVersionSettings
 import org.jetbrains.kotlin.config.languageVersionSettings
 import org.jetbrains.kotlin.diagnostics.*
@@ -73,7 +73,7 @@ class AnalyzerWithCompilerReport(
                     .append(", unresolved supertypes: ").append(unresolved!!.joinToString())
                     .append("\n")
             }
-            if (!languageVersionSettings.supportsFeature(LanguageFeature.ExtendedCompilerChecks)) {
+            if (!languageVersionSettings.getFlag(AnalysisFlags.extendedCompilerChecks)) {
                 message.append("Adding -Xextended-compiler-checks argument might provide additional information.\n")
             }
             messageCollector.report(ERROR, message.toString())