Enable ClangCodeModel plugin but add performance heuristics

Users do not know about the dual code model in Qt Creator, "fast and wrong" versus "slow and correct (+ features like warnings and annotations)".

Otherwise many issues with the clang code model (especially on Windows) are fixed now.

The "slow" part of the clang code model should actually be easy to detect automatically (even if how "slow" is "too slow" might not be so clear).

A good compromise could be to enable the clang code model by default, but show an info bar (including "do not show again") in an editor if we detect that it is "too slow". In the sense of "Code completion seems to be slow, you might want to try switching to the faster but imprecise built-in code model if that is an issue.". We also have the "details" functionality where we can be more detailed.