cmInstallGenerator: Move GetDestination to subclasses that need it

The method is used only for EXPORT and TARGET install destinations.
While at it, make it return a std::string by reference instead of a
"const char*".

Source/cmInstallGenerator.h

@@ -50,9 +50,6 @@ public:
     Indent const& indent = Indent()
     );
 
-  const char* GetDestination() const
-    { return this->Destination.c_str(); }
-
   /** Get the install destination as it should appear in the
       installation script.  */
   std::string GetInstallDestination() const;