C# syntax highlight glitch when using Tuple return type

Originally found in Github.com and also VSCode.

## Details

VSCode 1.34

## Repro

```csharp
public class {
public async Task<
(bool success,
string content)> Func(
ProductModel product,
Settings settings)
{
req1.Headers.Add("Accept", "application/json, text/javascript, */*; q=0.01");
req1.Headers.Add("Referer", $"{ShopBaseDomain}/shop/gd/dtl/goos.do?onlnGoosCd={goosCd}");
}
}
```