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/plugins/cpptools/cppcompletion_test.cpp

@@ -2689,6 +2689,19 @@ void CppToolsPlugin::test_completion_data()
         << QLatin1String("f")
         << QLatin1String("and_type"));
 
+    QTest::newRow("specialization_with_default_value") << _(
+            "class Foo {};\n"
+            "template<class T1 = Foo> class Temp;\n"
+            "template<> class Temp<Foo> { int var; };\n"
+            "void func()\n"
+            "{\n"
+            "    Temp<> t;\n"
+            "    @\n"
+            "}\n"
+     ) << _("t.") << (QStringList()
+        << QLatin1String("var")
+        << QLatin1String("Temp"));
+
     QTest::newRow("auto_declaration_in_if_condition") << _(
             "struct Foo { int bar; };\n"
             "void fun() {\n"