Add custom macro support for Clang Code Model

Add support for defining custom macros for Clang Code Model. Using custom project manager enables this using projectname.config file but CMake and QMake projects lack this functionality. When using Qt Creator's own code model Qt Creators shows a # button for defining custom macros. This will help correctly parsing the code in large projects like KDE. Please make this functionality also available for Clang Code Model.
Thanks

Edit: fixed core model -> code model