C++: Ignore explicit template instantiations

Defined in section 14.7.2 of the standard.

Fixes completion for std::string.

The following explicit instantiation appears in bits/basic_string.tcc:
  extern template class basic_string<char>;

This is wrongfully considered a specialization for a forward declaration
(like `template<> class basic_string<char>` is).

Introduce a new Symbol type for explicit instantiations.

Use-case:
template<class T>
struct Foo { T bar; };

template class Foo<int>;

void func()
{
    Foo<int> foo;
    foo.bar; // bar not highlighted
}

Change-Id: I9e35c8c32f6b78fc87b4f4f1fc903b42cfbd2c2b
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/libs/cplusplus/LookupContext.h

@@ -178,6 +178,7 @@ protected:
     void process(Symbol *root);
 
     virtual bool visit(Template *templ);
+    virtual bool visit(ExplicitInstantiation *inst);
     virtual bool visit(Namespace *ns);
     virtual bool visit(Class *klass);
     virtual bool visit(ForwardClassDeclaration *klass);