VS: Avoid empty, unreferenced solution folders... (#13571)

...in generated sub-directory sln files.

Thanks to rlandert for the bug report and proposed patch.

The method WriteTargetsToSolution gets called possibly multiple times,
once per sln file, (-> once per "project" command).

Before accumulating folder names in VisualStudioFolders, clear it
first, so it doesn't have stale entries in it from the previous
sln file.

Source/cmGlobalVisualStudio7Generator.cxx

@@ -268,6 +268,8 @@ void cmGlobalVisualStudio7Generator::WriteTargetsToSolution(
     cmLocalGenerator* root,
     OrderedTargetDependSet const& projectTargets)
 {
+  VisualStudioFolders.clear();
+
   for(OrderedTargetDependSet::const_iterator tt =
         projectTargets.begin(); tt != projectTargets.end(); ++tt)
     {