VS: Add toolset v142 CSharp flag table

While the flag tables for C and C++ were generated from MSBuild `.xml`
files, the CSharp flag tables were written by hand.  Copy the `v141`
flag table to use for the `v142` toolset.

Remove the special case added by commit 626c51f4 (VS: Update for
Visual Studio 2019 Preview 2, 2019-01-24, v3.14.0-rc1~74^2) that mapped
the v142 flag table lookup to v141 since we now have the real v142

Fixes: #19828
6 jobs for vs-v142-csharp-flags