cmake: introduce a cache of files with no compilation data

The following debug message is now printed once rather than 6 times for each file without compilation data:

kdevelop.plugins.cmake: no information found for "/path/to/file"

The added mutable data member should not break thread safety, because there is no locking when the data is modified. Apparently CMakeManager::fileInformation() is always called from the main thread. The definitions of DefinesAndIncludesManager's member functions that (indirectly) call fileInformation() start with the following assertion:

Q_ASSERT(QThread::currentThread() == qApp->thread());