Work around differences in clang libc++ std::vector<bool>::const_reference

clang++ using -stdlib=libc++ defines const_reference as a class,
to map boolean vectors onto a bit set.  Because swig does
not "see" the type as "const &" it generates incorrect code for this case,
generating a declaration like:

  const_reference result;

When const_reference is a typedef to 'bool' as is the case with stdlibc++
this works.  When this is actually a constant reference, this is clearly
invalid since it is not initialized.  For libc++, this is a class
which cannot be default constructed, resulting in an error.  The fix
is to explicitly define the various accessor extensions as having a
bool return type for this specialization.

Lib/csharp/std_vector.i

@@ -217,7 +217,7 @@
         else
           throw std::out_of_range("index");
       }
-      const_reference getitem(int index) throw (std::out_of_range) {
+      CONST_REFERENCE getitem(int index) throw (std::out_of_range) {
         if (index>=0 && index<(int)$self->size())
           return (*$self)[index];
         else