[FIX] Add a checker for unsupported explicit backing fields

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/CommonDeclarationCheckers.kt

@@ -65,6 +65,7 @@ object CommonDeclarationCheckers : DeclarationCheckers() {
     override val backingFieldCheckers: Set<FirBackingFieldChecker>
         get() = setOf(
             FirExplicitBackingFieldForbiddenChecker,
+            FirExplicitBackingFieldsUnsupportedChecker,
         )
 
     override val classCheckers: Set<FirClassChecker>

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirExplicitBackingFieldsUnsupportedChecker.kt

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.fir.analysis.checkers.declaration
+
+import org.jetbrains.kotlin.config.LanguageFeature
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.analysis.diagnostics.DiagnosticReporter
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors
+import org.jetbrains.kotlin.fir.analysis.diagnostics.reportOn
+import org.jetbrains.kotlin.fir.declarations.FirBackingField
+import org.jetbrains.kotlin.fir.declarations.impl.FirDefaultPropertyBackingField
+import org.jetbrains.kotlin.fir.languageVersionSettings
+
+object FirExplicitBackingFieldsUnsupportedChecker : FirBackingFieldChecker() {
+    override fun check(declaration: FirBackingField, context: CheckerContext, reporter: DiagnosticReporter) {
+        if (declaration !is FirDefaultPropertyBackingField &&
+            !context.session.languageVersionSettings.supportsFeature(LanguageFeature.ExplicitBackingFields)
+        ) {
+            reporter.reportOn(
+                declaration.source,
+                FirErrors.UNSUPPORTED_FEATURE,
+                LanguageFeature.ExplicitBackingFields to context.session.languageVersionSettings,
+                context
+            )
+        }
+    }
+}