[FIR] Forbid multiple labels per statement

^KT-53629: Fixed

compiler/fir/tree/src/org/jetbrains/kotlin/fir/diagnostics/ConeSimpleDiagnostic.kt

@@ -27,6 +27,10 @@ class ConeUnderscoreIsReserved(source: KtSourceElement) : ConeDiagnosticWithSour
     override val reason: String get() = "Names _, __, ___, ..., are reserved in Kotlin"
 }
 
+class ConeMultipleLabelsAreForbidden(source: KtSourceElement) : ConeDiagnosticWithSource(source) {
+    override val reason: String get() = "Multiple labels per statement are forbidden"
+}
+
 class ConeCannotInferTypeParameterType(
     val typeParameter: FirTypeParameterSymbol,
     override val reason: String = "Cannot infer type for parameter ${typeParameter.name}"

compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirExpressionUtil.kt

@@ -102,7 +102,7 @@ fun <T : FirStatement> FirBlock.replaceFirstStatement(factory: (T) -> FirStateme
 }
 
 fun FirExpression.unwrapErrorExpression(): FirExpression? =
-    if (this is FirErrorExpression) expression?.run { unwrapErrorExpression() } else this
+    if (this is FirErrorExpression) expression?.unwrapErrorExpression() else this
 
 fun FirExpression.unwrapArgument(): FirExpression = (this as? FirWrappedArgumentExpression)?.expression ?: this