[natvis] gsl::span<T, gsl::dynamic_extent> shows `{ extent = 4294967295 }` in debugger

https://github.com/microsoft/GSL/blob/226a854a97087bbd9aa5ec2ccf94902b66037a72/GSL.natvis#L24

This doesn't seem to work in latest vs 2019:
![image](https://user-images.githubusercontent.com/1614246/77729483-f898fd00-7028-11ea-96e6-80af697fc652.png)