[FIR] Forbid explicit backing fields in extension properties

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

@@ -2442,6 +2442,12 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.EXPLICIT_BACKING_FIELD_IN_EXTENSION) { firDiagnostic ->
+        ExplicitBackingFieldInExtensionImpl(
+            firDiagnostic as FirPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.ABSTRACT_PROPERTY_IN_PRIMARY_CONSTRUCTOR_PARAMETERS) { firDiagnostic ->
         AbstractPropertyInPrimaryConstructorParametersImpl(
             firDiagnostic as FirPsiDiagnostic,

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

@@ -1727,6 +1727,10 @@ sealed class KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = ExplicitBackingFieldInAbstractProperty::class
     }
 
+    abstract class ExplicitBackingFieldInExtension : KtFirDiagnostic<KtBackingField>() {
+        override val diagnosticClass get() = ExplicitBackingFieldInExtension::class
+    }
+
     abstract class AbstractPropertyInPrimaryConstructorParameters : KtFirDiagnostic<KtModifierListOwner>() {
         override val diagnosticClass get() = AbstractPropertyInPrimaryConstructorParameters::class
     }

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

@@ -2818,6 +2818,13 @@ internal class ExplicitBackingFieldInAbstractPropertyImpl(
     override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
 }
 
+internal class ExplicitBackingFieldInExtensionImpl(
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.ExplicitBackingFieldInExtension(), KtAbstractFirDiagnostic<KtBackingField> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
 internal class AbstractPropertyInPrimaryConstructorParametersImpl(
     firDiagnostic: FirPsiDiagnostic,
     override val token: ValidityToken,