Move Visual Studio solution to root directory.

Change-Id: Ifb474e6ab12b254599b6d1713f9f2a01b869e09d
Reviewed-on: https://swiftshader-review.googlesource.com/5361
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>