VS: Compare VS instance versions as strings

This makes the values more readable.
diff --git a/Source/cmGlobalVisualStudio10Generator.h b/Source/cmGlobalVisualStudio10Generator.h
index 48fe465..646dbe2 100644
--- a/Source/cmGlobalVisualStudio10Generator.h
+++ b/Source/cmGlobalVisualStudio10Generator.h
@@ -128,10 +128,7 @@
   std::string Encoding() override;
   const char* GetToolsVersion() const;
 
-  virtual cm::optional<unsigned long long> GetVSInstanceVersion() const
-  {
-    return {};
-  }
+  virtual cm::optional<std::string> GetVSInstanceVersion() const { return {}; }
 
   bool GetSupportsUnityBuilds() const { return this->SupportsUnityBuilds; }