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/3rdparty/cplusplus/Templates.cpp

@@ -125,6 +125,12 @@ void CloneType::visit(Template *type)
     _type = templ;
 }
 
+void CloneType::visit(ExplicitInstantiation *type)
+{
+    ExplicitInstantiation *inst = _clone->symbol(type, _subst)->asExplicitInstantiation();
+    _type = inst;
+}
+
 void CloneType::visit(Class *type)
 {
     Class *klass = _clone->symbol(type, _subst)->asClass();
@@ -291,6 +297,14 @@ bool CloneSymbol::visit(Template *symbol)
     return false;
 }
 
+bool CloneSymbol::visit(ExplicitInstantiation *symbol)
+{
+    ExplicitInstantiation *inst = new ExplicitInstantiation(_clone, _subst, symbol);
+    _symbol = inst;
+    _control->addSymbol(inst);
+    return false;
+}
+
 bool CloneSymbol::visit(Class *symbol)
 {
     Class *klass = new Class(_clone, _subst, symbol);