C++: fix the quick-exit for already parsed files.

When the preprocessor is asked to process a file which it has already
seen before, it can skip it, but it still has to add the file to the
list of included files for the current document.

Task-number: QTCREATORBUG-9107
Change-Id: I30cac61c1da20da7fe925168ecccb429584e9746
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>