FIR: make ConeVariableExpectedError an object

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/diagnostics/FirDiagnostics.kt

@@ -131,7 +131,7 @@ class ConeOperatorAmbiguityError(override val candidateSymbols: Collection<FirBa
     override val reason: String get() = "Operator overload ambiguity. Compatible candidates: ${candidateSymbols.map { describeSymbol(it) }}"
 }
 
-class ConeVariableExpectedError : ConeDiagnostic {
+object ConeVariableExpectedError : ConeDiagnostic {
     override val reason: String get() = "Variable expected"
 }
 

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/FirExpressionsResolveTransformer.kt

@@ -528,7 +528,7 @@ open class FirExpressionsResolveTransformer(transformer: FirBodyResolveTransform
                                     leftArgument.calleeReference.source
                                 else -> leftArgument.source
                             }
-                            diagnostic = if (lhsSymbol == null) ConeVariableExpectedError() else ConeValReassignmentError(lhsSymbol)
+                            diagnostic = if (lhsSymbol == null) ConeVariableExpectedError else ConeValReassignmentError(lhsSymbol)
                         }
                     }
                     (leftArgument as? FirQualifiedAccess)?.let {