Clang: Add sorting by priority

The priority is adjusted too provide a better completion list.

Change-Id: I1ebed1996f660046843e0c5249a91e8c2b1eeb88
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/plugins/clangcodemodel/clangassistproposalmodel.h

@@ -48,6 +48,7 @@ public:
     {}
 
     bool isSortable(const QString &prefix) const override;
+    void sort(const QString &prefix) override;
 
     static bool replaceDotForArrow(IAssistProposalModel *model);