C++: fix code completion for decltyped type

example:
struct Foo { int bar; };
Foo foo() { return Foo; }
typedef decltype(foo()) TypedefedFooWithDecltype;
void fun()
{
  decltype(foo()) decltypeFoo;
  decltypeFoo.;// code completion should work here

  TypedefedFooWithDecltype typedefedFooWithDecltype;
  typedefedFooWithDecltype.;// code completion should work here
}

Started-by: Przemyslaw Gorszkowski <pgorszkowski@gmail.com>
Task-number: QTCREATORBUG-14483
Change-Id: I296ceed9d896c68cf0651265afb08a1fc42f9a68
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/libs/cplusplus/LookupContext.h

@@ -41,6 +41,7 @@
 #include <cplusplus/Control.h>
 #include <cplusplus/Name.h>
 
+#include <QEnableSharedFromThis>
 #include <QSet>
 #include <QMap>
 
@@ -99,7 +100,9 @@ private:
     friend class CreateBindings;
 };
 
-class CPLUSPLUS_EXPORT CreateBindings: protected SymbolVisitor
+class CPLUSPLUS_EXPORT CreateBindings
+        : protected SymbolVisitor
+        , public QEnableSharedFromThis<CreateBindings>
 {
     Q_DISABLE_COPY(CreateBindings)
 
@@ -119,6 +122,9 @@ public:
     QSharedPointer<Control> control() const
     { return _control; }
 
+    Snapshot &snapshot()
+    { return _snapshot; }
+
     /// Adds an expression document in order to keep their symbols and names alive
     void addExpressionDocument(Document::Ptr document)
     { _expressionDocuments.append(document); }