C++: Fix expensive lookup for boost

With this patch, CheckSymbols takes about 200ms for processing the boost/proto
hello world example [1]. Before, it needed about 11 seconds (timer in
CheckSymbols::run). Same goes for including <boost/fusion/include/zip.hpp>.

The custom ProcessedSet object was added to support "completion for typedefs
for templates in namespaces", but apparently that's not needed anymore, as the
added test proves.

[1] http://www.boost.org/doc/libs/1_58_0/doc/html/proto/users_guide.html#boost_proto.users_guide.getting_started.hello_world

Task-numer: QTCREATORBUG-14889
Task-numer: QTCREATORBUG-14741
Change-Id: I90454e8970a9d04033d56beeb0c6d7a0d4e6cc62
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/cpptools/cppcompletion_test.cpp

@@ -3235,6 +3235,24 @@ void CppToolsPlugin::test_completion_data()
             << QLatin1String("Foo")
             << QLatin1String("bar"));
 
+    QTest::newRow("typedef for templates in namespace") << _(
+             "namespace N {\n"
+             "\n"
+             "struct Data { int x; };\n"
+             "template <typename T> struct Foo { T member; };\n"
+             "typedef Foo<Data> Bar;\n"
+             "\n"
+             "} // N\n"
+             "\n"
+             "\n"
+             "void f()\n"
+             "{\n"
+             "    N::Bar o;\n"
+             "    @\n"
+             "}\n"
+    ) << _("o.member.") << (QStringList()
+            << QLatin1String("Data")
+            << QLatin1String("x"));
 }
 
 void CppToolsPlugin::test_completion_member_access_operator()