cmLocalGenerator: add an override for CMakeFiles-using paths

Visual Studio doesn't use a `CMakeFiles` subdirectory for its support
directories. However, some codepaths expect to use paths which are
always under `CMakeFiles`. Add a mechanism to keep the path for such
files.

Source/cmLocalGenerator.h

@@ -439,6 +439,7 @@ public:
   std::string const& GetCurrentSourceDirectory() const;
 
   virtual std::string GetObjectOutputRoot() const;
+  virtual bool AlwaysUsesCMFPaths() const;
 
   /**
    * Generate a macOS application bundle Info.plist file.