Provide -cachestorepath or -clearcachestore commandline option

In a bug free world, the recently introduced CacheStore would just do its job in the background and everything would be fine.

But a couple of places in Qt Creator still stumble upon this feature (e.g. QTCREATORBUG-30490). In order to diagnose that, users currently need to manually find and delete the cache. -tcs does not invalidate the cache, since the CacheStore is separated from the settings.

Having an option such as -cachestorepath, or -clearcachestore which on startup clears the whole store could be a remedy.

-clearcachestore is less ideal, since Qt Creator instances with separate user settings location interfere (fore example Qt Creator which I work with and the Qt Creator that I work on share the cache), but better than manually finding and deleting the path.