ClangCodeModel should cache its result

Whenever CMake is run (or the qmake project is reparsed), Qt Creator starts reparsing everything. In large projects (actually, in any non-trivial project), this takes quite a bit of time.

On the code base I work on, this takes around 10 minutes.

Every time I add/remove/rename a file, CMake reruns triggering a full reparse, and I wait another 10 minutes.

Every time I switch sessions, CMake reruns triggering a full reparse, and I wait another ten minutes.

Even though only incremental changes happen (a file is added/removed/renamed) or nothing happens at all.

The result of a parse should be stored on disk and linked to a Qt Creator session per project. Currently the parse result only exists in memory, and is completely regenerated every time a (small) thing in the project changes.

I'd be happy that on a clean load or session switch, the previous parse result is just loaded. This may be related to the unecessary CMake run when loading a project that has previously been loaded before. It's just a waste of time and should be disabled.