Clang: Use new libclang code completion fix-its feature

Each completion coming from libclang now has it's own
list of fix-its which are required to be applied before
completion itself.

This saves one extra reparse cycle and gives an ability
to provide both kinds of completion (initial and corrected
one) for cases like shared_ptr, unique_ptr or any other
class with overloaded arrow operator.

Each of these extra fix-its is applied together with
corresponding completion dircetly before completion itself.

Change-Id: Ide37e45bb15fa2f1375cd6b86ecd43ced3593046
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/texteditor/codeassist/assistproposaliteminterface.h

@@ -65,6 +65,7 @@ public:
     virtual bool isSnippet() const = 0;
     virtual bool isValid() const = 0;
     virtual quint64 hash() const = 0; // it is only for removing duplicates
+    virtual bool requiresFixIts() const { return false; }
 
     inline int order() const { return m_order; }
     inline void setOrder(int order) { m_order = order; }