Once CMake supports VS2019 as the project generator use it if installed

[safern]

In: dotnet/corefx#34381 we enabled building and generating the native project with VS2019, but CMake doesn't support VS2019 as the project generator. So we reverted the generator change in: dotnet/corefx#34412

This issue is to track CMake's support for VS2019 and to update the generator script.

cc: @ViktorHofer

[safern]

There is already a PR in cmake to add the 2019 generator: https://gitlab.kitware.com/cmake/cmake/merge_requests/2711

[safern]

PR was replaced with: https://gitlab.kitware.com/cmake/cmake/merge_requests/2789

It was merged 2 days ago and it is marked for 3.14 cmake version.