C++: Fix resolving of partial specialization

Use-case:
struct b {};
template<class X, class Y> struct s { float f; };
template<class X> struct s<X, b> { int i; };

void f()
{
    s<int, b> var;
    var.i; // i not highlighted
}

Task-number: QTCREATORBUG-14036
Change-Id: I70a87499e0a375e84d992ca0a79d77270a3419e8
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/libs/cplusplus/LookupContext.cpp

@@ -1212,6 +1212,9 @@ LookupScopePrivate *LookupScopePrivate::findSpecialization(
                         }
                     }
                 }
+
+                if (specializationTemplateArgument == initializationTemplateArgument)
+                    return cit->second;
             }
         }
     }

src/plugins/cpptools/cppcompletion_test.cpp

@@ -2623,6 +2623,20 @@ void CppToolsPlugin::test_completion_data()
     ) << _("s.") << (QStringList()
         << QLatin1String("S"));
 
+    QTest::newRow("partial_specialization") << _(
+            "struct b {};\n"
+            "template<class X, class Y> struct s { float f; };\n"
+            "template<class X> struct s<X, b> { int i; };\n"
+            "\n"
+            "void f()\n"
+            "{\n"
+            "    s<int, b> var;\n"
+            "    @\n"
+            "}\n"
+    ) << _("var.") << (QStringList()
+        << QLatin1String("i")
+        << QLatin1String("s"));
+
     QTest::newRow("auto_declaration_in_if_condition") << _(
             "struct Foo { int bar; };\n"
             "void fun() {\n"