fix watson bug

oop serialization fails when invalid preprocessor is given to serializer.

either compiler prevent such parse option being created or serializer should be able to create all options that already exist

src/Workspaces/CSharp/Portable/Execution/CSharpOptionsSerializationService.cs

@@ -84,8 +84,12 @@ public override ParseOptions ReadParseOptionsFrom(ObjectReader reader, Cancellat
             var languageVersion = (LanguageVersion)reader.ReadInt32();
             var preprocessorSymbolNames = reader.ReadArray<string>();
 
-            var options = new CSharpParseOptions(languageVersion, documentationMode, kind, preprocessorSymbolNames);
-            return options.WithFeatures(features);
+            var options = new CSharpParseOptions(languageVersion, documentationMode, kind);
+
+            // use WithPreprocessorSymbols instead of constructor to bypass preprocessor validation.
+            // https://github.com/dotnet/roslyn/issues/15797
+            return options.WithPreprocessorSymbols(preprocessorSymbolNames)
+                          .WithFeatures(features);
         }
     }
 }