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/Control.cpp

@@ -366,9 +366,16 @@ public:
 
     Template *newTemplate(unsigned sourceLocation, const Name *name)
     {
-        Template *ns = new Template(translationUnit, sourceLocation, name);
-        symbols.push_back(ns);
-        return ns;
+        Template *templ = new Template(translationUnit, sourceLocation, name);
+        symbols.push_back(templ);
+        return templ;
+    }
+
+    ExplicitInstantiation *newExplicitInstantiation(unsigned sourceLocation, const Name *name)
+    {
+        ExplicitInstantiation *inst = new ExplicitInstantiation(translationUnit, sourceLocation, name);
+        symbols.push_back(inst);
+        return inst;
     }
 
     NamespaceAlias *newNamespaceAlias(unsigned sourceLocation, const Name *name)
@@ -692,6 +699,9 @@ Namespace *Control::newNamespace(unsigned sourceLocation, const Name *name)
 Template *Control::newTemplate(unsigned sourceLocation, const Name *name)
 { return d->newTemplate(sourceLocation, name); }
 
+ExplicitInstantiation *Control::newExplicitInstantiation(unsigned sourceLocation, const Name *name)
+{ return d->newExplicitInstantiation(sourceLocation, name); }
+
 NamespaceAlias *Control::newNamespaceAlias(unsigned sourceLocation, const Name *name)
 { return d->newNamespaceAlias(sourceLocation, name); }