C++: Fix decltype resolving for template function

The last nail for std::unique_ptr (GCC variant, MSVC still doesn't work).

Use-case:
template<typename T>
static T f();

struct Foo { int bar; };

void fun()
{
    decltype(f<Foo>()) s;
    s.bar; // bar not highlighted
}

Task-number: QTCREATORBUG-14483
Task-number: QTCREATORBUG-8937
Change-Id: I5bab757400b070cf9dbb688a44fd8eafe95ddc61
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/plugins/cpptools/cppcompletion_test.cpp

@@ -2984,6 +2984,21 @@ void CppToolsPlugin::test_completion_data()
     ) << _("s.") << (QStringList()
             << QLatin1String("Foo")
             << QLatin1String("bar"));
+
+    QTest::newRow("typedefed_decltype_of_template_function") << _(
+            "template<typename T>\n"
+            "static T f();\n"
+            "\n"
+            "struct Foo { int bar; };\n"
+            "\n"
+            "void fun()\n"
+            "{\n"
+            "    decltype(f<Foo>()) s;\n"
+            "    @\n"
+            "}\n"
+    ) << _("s.") << (QStringList()
+            << QLatin1String("Foo")
+            << QLatin1String("bar"));
 }
 
 void CppToolsPlugin::test_completion_member_access_operator()