FIR checker: rework VAL_REASSIGNMENT_VIA_BACKING_FIELD(_ERROR)

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/checkers/CommonExpressionCheckers.kt

@@ -25,12 +25,17 @@ object CommonExpressionCheckers : ExpressionCheckers() {
         FirTypeParameterInQualifiedAccessChecker,
         FirSealedClassConstructorCallChecker,
     )
+
     override val functionCallCheckers: Set<FirFunctionCallChecker> = setOf()
 
     override val tryExpressionCheckers: Set<FirTryExpressionChecker> = setOf(
         FirCatchParameterChecker
     )
 
+    override val variableAssignmentCheckers: Set<FirVariableAssignmentChecker> = setOf(
+        FirValReassignmentViaBackingFieldChecker
+    )
+
     override val whenExpressionCheckers: Set<FirWhenExpressionChecker> = setOf(
         FirExhaustiveWhenChecker
     )