Move to generating msbuild choose elements to get if-else selection behavior for sources in projects so that we don't get erroneous duplicate files in cases where there are both profile and host platform criteria (#9952)
A recent commit revealed that in cases where we select based on a mix of host platform and profile, genproj csproj files can end up with duplicate sources because the existing <ItemGroup Condition= approach could make multiple groups match for a given compile when we really just want one.
This PR changes to generating a cascade of msbuild <Choose> elements, which give if-else selection to ensure that we only ever build a single set of files.