THRIFT-1188 C++: Use bit fields for a more compact __isset struct

Patch: Eric Rannaud
diff --git a/compiler/cpp/src/generate/t_cpp_generator.cc b/compiler/cpp/src/generate/t_cpp_generator.cc
index bd38ff9..3a190f5 100755
--- a/compiler/cpp/src/generate/t_cpp_generator.cc
+++ b/compiler/cpp/src/generate/t_cpp_generator.cc
@@ -934,7 +934,7 @@
     for (m_iter = members.begin(); m_iter != members.end(); ++m_iter) {
       if ((*m_iter)->get_req() != t_field::T_REQUIRED) {
         indent(out) <<
-          "bool " << (*m_iter)->get_name() << ";" << endl;
+          "bool " << (*m_iter)->get_name() << " :1;" << endl;
         }
       }