vsimporter shouldn't add empty groups to proj files (#2433)
vsimporter shouldn't add empty groups to proj files (#2433)

Groups that are vsimporter specific and empty should not be added to the
generated project file. However, generic VS groups like "ExtensionTargets"
and "Shared" should stay.

Fixes #2054
bbowman/WinObjC,ehren/WinObjC,bbowman/WinObjC,ehren/WinObjC,pradipd/WinObjC,pradipd/WinObjC,ehren/WinObjC,nathpete-msft/WinObjC,nathpete-msft/WinObjC,nathpete-msft/WinObjC,bbowman/WinObjC,ehren/WinObjC,pradipd/WinObjC,nathpete-msft/WinObjC,bbowman/WinObjC,ehren/WinObjC,pradipd/WinObjC,pradipd/WinObjC,bbowman/WinObjC,nathpete-msft/WinObjC