Some links about design-time builds:
- https://github.com/dotnet/project-system/blob/master/docs/design-time-builds.md
- https://daveaglick.com/posts/running-a-design-time-build-with-msbuild-apis
In Visual Studio 2017 (Windows), a "design-time" build is what drives
Intellisense in cases such as using `x:Name` in XAML (or
`Resource.designer.cs` in a plain Xamarin.Android app). Improving our
"design-time" build speed will improve general performance in the IDE
such as project load and how long Intellisense takes to update. Since
XamlC is not needed during a design-time build, it is an "easy-win" to
just disable it. XamlC can take a bit of time for projects with a lot
of XAML.
This might also help with: https://github.com/xamarin/Xamarin.Forms/issues/3004
Although we should probably not do anything to specifically support
NCrunch; hopefully, NCrunch is running under the context of a
design-time build.
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