Memory Leak at reloading project

Steps to reproduce:

1. Open qtcreator project source with qtcreator, let it finish parsing c/c++ symbols. Note the memory usage.
2. Close qtcreator project. Don't close qtcreator. Note the memory usage doesn't reduce.
3. Open qtcreator project source again. Note the memory usage doubled compare to the first time.

This issue may be trivial on small code base project. But for project that has millions lines of code, every reload consumes 1~2GB of memory. The steps are one of the way to reproduce it. There are other means of reproduction such as right click project and edit files will also cause project reload and symbols parsing.