[cplusplus] file open/parsing bottleneck in filesystem access

when opening a file the preprocessor is executed (`Preprocessor::run`). with my codebase more than 20% of the time is spent in the `stat` system call, inside `CppSourceProcessor::checkFile(QString const&)` -> `QFileInfo::isFile()`

makes me wonder if there's any way to avoid the system call, e.g. by using a local cache in combination with a filesystem watcher