Update PopulateSwitchStatementHelpers.cs

Define out variable

src/Features/Core/Portable/PopulateSwitch/PopulateSwitchStatementHelpers.cs

@@ -51,7 +51,7 @@ public static ICollection<ISymbol> GetMissingEnumMembers(ISwitchOperation switch
             // Check if the type of the expression is a nullable INamedTypeSymbol
             // if the type is both nullable and an INamedTypeSymbol extract the type argument from the nullable
             // and check if it is of enum type
-            switchExpressionType = switchExpressionType.IsNullable(out underlyingType) ? underlyingType : switchExpressionType;
+            switchExpressionType = switchExpressionType.IsNullable(out var underlyingType) ? underlyingType : switchExpressionType;
 
             if (switchExpressionType?.TypeKind == TypeKind.Enum)
             {