Call the generator "Visual Studio 15" without any year because the
preview version of VS 15 does not provide a year in the product name.
Copy cmGlobalVisualStudio14Generator to cmGlobalVisualStudio15Generator
and update version numbers accordingly. Add the VS15 enumeration value.
Note that we do not need to add a MSVC15 variable or v150 toolset
because Visual Studio 15 comes with an updated version of the v140
toolset and remains ABI-compatible.
Teach tests VSExternalInclude, RunCMake.GeneratorPlatform, and
RunCMake.GeneratorToolset to treat VS 15 as they do VS 10-14.