C++: Support default template argument lookup for specialization

This fixes std::vector, although it doesn't really resolve numeric
template arguments. It just picks the first specialization.

Use-case:
class Foo {};
template<class T1 = Foo> class Temp;
template<> class Temp<Foo> { int var; };
void func()
{
    Temp<> t;
    t.var; // var not highlighted
}

Task-number: QTCREATORBUG-8922
Change-Id: I593515beb3a6d901b6088db8bc1b8e16c39083d3
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/libs/cplusplus/LookupContext.h

@@ -150,8 +150,13 @@ public:
     /// \internal
     LookupScope *allocLookupScope(LookupScope *parent, const Name *name);
 
+    FullySpecifiedType resolveTemplateArgument(Clone &cloner, Subst &subst,
+                                               LookupScope *origin,
+                                               const Template *specialization,
+                                               const TemplateNameId *instantiation,
+                                               unsigned index);
     void initializeSubst(Clone &cloner, Subst &subst, LookupScope *origin,
-                         Template *specialization, const TemplateNameId *instantiation);
+                         const Template *specialization, const TemplateNameId *instantiation);
 
 protected:
     using SymbolVisitor::visit;