Figure out upgrade

[davkean]

From @davkean on May 13, 2016 2:10

While most projects don't need to upgrade from VS 2015 -> VS 15, we still have to handle project upgrade from earlier versions.

We basically need to fill out implementations of IFlavoredProjectCompatibilityProvider.IsProjectNeedBeUpgradedAsync to decide whether to delegate onto csproj.

Copied from original issue: dotnet/roslyn#11285