GitLab

* Cuts direct dependency to CPlusPlus from cppeditor.h, but
  cppfunctiondecldeflink.h still pulls it in.

* Cuts direct dependency to
  cppeditor.h from cppvirtualfunctionassistprovider.cpp, but it still
  depends on cppeditorconstants.h.

* Cuts direct dependency to cppeditor.h from cppelementevaluator.cpp.

The long-term goal is to make the CppEditor independent from concrete
code model backends.

Change-Id: I291ee0d0da5fc5ed1a839a763fe7be11dcf7a6fb
Reviewed-by: Marco Bubke <marco.bubke@theqtcompany.com>