When a CSharp target links to a static C++ library, CMake will compute
the link language as C++ instead of CSharp.  That may be incorrect and
needs further investigation, but it does not affect how VS drives C#
linking.  However, it does break our flag language selection logic
and causes C++ flags to be used for CSharp.  In particular, this
drops the `-platform:x86` flag on 32-bit builds.

Fix this by always selecting the CSharp flags when generating a
`.csproj` project type.

Issue: #18239