Enable new experimental:preprocessor option in Visual Studio if possible

The new conformant preprocessor is feature complete in Visual Studio 16.5.

https://docs.microsoft.com/en-us/cpp/preprocessor/preprocessor-experimental-overview?view=vs-2019

Try updating to use it instead of the traditional preprocessor (which is well known to be non-conforming in various ways). This would bring VS-based usage more in line with other platforms in this area, and remove the need for some workarounds for the traditional preprocessor's behavior.