VS: Update CSharp flag table to support new language versions
This is based off of the latest 15.8.5 release of VS2017. CSC version 2.9.0.63208 (958f2354).
Edited by Brad King
This is based off of the latest 15.8.5 release of VS2017. CSC version 2.9.0.63208 (958f2354).