[FIR] Forbid backing fields for delegated properties

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt

@@ -2436,6 +2436,12 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.BACKING_FIELD_FOR_DELEGATED_PROPERTY) { firDiagnostic ->
+        BackingFieldForDelegatedPropertyImpl(
+            firDiagnostic as FirPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.PROPERTY_MUST_HAVE_GETTER) { firDiagnostic ->
         PropertyMustHaveGetterImpl(
             firDiagnostic as FirPsiDiagnostic,

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt

@@ -1723,6 +1723,10 @@ sealed class KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = LateinitNullableBackingField::class
     }
 
+    abstract class BackingFieldForDelegatedProperty : KtFirDiagnostic<KtBackingField>() {
+        override val diagnosticClass get() = BackingFieldForDelegatedProperty::class
+    }
+
     abstract class PropertyMustHaveGetter : KtFirDiagnostic<KtProperty>() {
         override val diagnosticClass get() = PropertyMustHaveGetter::class
     }

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt

@@ -2774,6 +2774,13 @@ internal class LateinitNullableBackingFieldImpl(
     override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
 }
 
+internal class BackingFieldForDelegatedPropertyImpl(
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.BackingFieldForDelegatedProperty(), KtAbstractFirDiagnostic<KtBackingField> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
 internal class PropertyMustHaveGetterImpl(
     firDiagnostic: FirPsiDiagnostic,
     override val token: ValidityToken,