Ice DLLs and PDBs not removed during clean

[pepone]

When you clean the "C# NET Core demos" solution in VisualStuio, the various Ice DLLs and PDBs copied next to each demos don't get cleaned. This is true for 3.7 and 3.6

Seems related to changing the project structure and have the output directory in the parent directory of the project directory. Created a test case and reported the issue to MSBuild project see dotnet/msbuild#3189