Use System.CommandLine for ilc.exe command line parsing

[jkotas]

https://github.com/dotnet/corefxlab/tree/master/src/System.CommandLine, CLI repo has number of examples

[tijoytom-zz]

@jkotas @gkhanna79 I switched to System.CommandLine for mcg and if you want i can change ilc too.For mcg i included the source files directly , is there a nuget feed for corefxlab i can use that.

[schellap]

I have it done this last night, so skip ILC please.

[gkhanna79]

.For mcg i included the source files directly

We should use nuget package for System.CommandLine. I spoke @schellap today who is doing similar work for ILC and can point you to the package to consume.

[schellap]

https://dotnet.myget.org/gallery/dotnet-corefxlab

The WIP diff is in schellap/corert/syscmd which uses it from the myget feed. I had to make it work for both desktop project and dnxcore50 project so I am using a slightly older version of the System.CommandLine.

[jkotas]

Fixed by #859