VS: Introduce IsInSolution to check whether a target is in sln file

Previously, different versions of VS Generators checked whether a target
was in .sln file or not by checking whether the target was to be written
to build system or not.

As we move `ZERO_CHECK.vcxproj` to `.proj`, we want to exclude those
files from being written to `.sln` files too. This commit introduces
`IsInSolution()` at `cmGlobalVisualStudioGenerator` level which can be
customized at specific versioned Generator when needed.

Source/cmGlobalVisualStudio7Generator.cxx

@@ -359,7 +359,7 @@ void cmGlobalVisualStudio7Generator::WriteTargetConfigurations(
   // loop over again and write out configurations for each target
   // in the solution
   for (cmGeneratorTarget const* target : projectTargets) {
-    if (!target->IsInBuildSystem()) {
+    if (!this->IsInSolution(target)) {
       continue;
     }
     cmValue expath = target->GetProperty("EXTERNAL_MSPROJECT");
@@ -396,7 +396,7 @@ void cmGlobalVisualStudio7Generator::WriteTargetsToSolution(
   VisualStudioFolders.clear();
 
   for (cmGeneratorTarget const* target : projectTargets) {
-    if (!target->IsInBuildSystem()) {
+    if (!this->IsInSolution(target)) {
       continue;
     }
     bool written = false;

Source/cmGlobalVisualStudio8Generator.cxx

@@ -392,7 +392,7 @@ void cmGlobalVisualStudio8Generator::WriteProjectDepends(
   TargetDependSet const& unordered = this->GetTargetDirectDepends(gt);
   OrderedTargetDependSet depends(unordered, std::string());
   for (cmTargetDepend const& i : depends) {
-    if (!i->IsInBuildSystem()) {
+    if (!this->IsInSolution(i)) {
       continue;
     }
     std::string guid = this->GetGUID(i->GetName());

Source/cmGlobalVisualStudioGenerator.cxx

@@ -837,6 +837,12 @@ bool cmGlobalVisualStudioGenerator::TargetIsFortranOnly(
   return languages.size() == 1 && *languages.begin() == "Fortran";
 }
 
+bool cmGlobalVisualStudioGenerator::IsInSolution(
+  const cmGeneratorTarget* gt) const
+{
+  return gt->IsInBuildSystem();
+}
+
 bool cmGlobalVisualStudioGenerator::TargetCompare::operator()(
   cmGeneratorTarget const* l, cmGeneratorTarget const* r) const
 {

Source/cmGlobalVisualStudioGenerator.h

@@ -98,6 +98,9 @@ public:
   // return true if target is fortran only
   bool TargetIsFortranOnly(const cmGeneratorTarget* gt);
 
+  // return true if target should be included in solution.
+  virtual bool IsInSolution(const cmGeneratorTarget* gt) const;
+
   /** Get the top-level registry key for this VS version.  */
   std::string GetRegistryBase();