Include [ComVisible(false)] in generated _AssemblyInfo.cs file

[stephentoub]

Per the discussion at dotnet/corefx#782, we're scrubbing [ComVisible] attribution from the CoreFX sources. If the resolution of that discussion is that we should have an [assembly: ComVisible(false)] attribute in every assembly, we should put it in the generated _AssemblyInfo.cs file that's generated as part of the build.

[weshaggard]

The support for this has been included in the build tools we now just need to decide whether or not we include it by default by setting AssemblyComVisible=false in our src\dir.targets in the CoreFX repo.

[weshaggard]

We have support for this in the build targets but right now we aren't putting any AssemblyComVisible attributes on our assemblies. I'm closing this issue as it was more about adding support for this if we feel we need to set it to false then we just need to set the AssemblyComVisible property to false in our dir.props for the repo.