Pre-compute object file names before VS project generation

Implement cmGlobalGenerator::ComputeTargetObjects in the VS generator
to pre-compute all the object file names.  Use the results during
generation instead of re-computing it later.

Source/cmLocalVisualStudio7Generator.h

@@ -60,11 +60,7 @@ public:
   virtual std::string GetTargetDirectory(cmTarget const&) const;
   cmSourceFile* CreateVCProjBuildRule();
   void WriteStampFiles();
-  // Compute the maximum length full path to the intermediate
-  // files directory for any configuration.  This is used to construct
-  // object file names that do not produce paths that are too long.
-  void ComputeMaxDirectoryLength(std::string& maxdir,
-                                 cmTarget& target);
+  virtual std::string ComputeLongestObjectDirectory(cmTarget&) const;
 
   virtual void ReadAndStoreExternalGUID(const char* name,
                                         const char* path);