[libc++] Fix GDB pretty printers when GDB uses Python 2.7

[arichardson]

Summary

The gdb_pretty_printer_test.sh fails if GDB was built against Python 2.7
since Python 2 expects iterators to have a next() method rather than
using next. To make the pretty printers work with both Python 2 and 3
we can simply set next to next in the iterator classes.

Python 2.7 support was removed in f46f93b4786377dd7ee704ef2beedadfe4f05adf,
so this effectively reverts that commit. While Python 2.7 is EOL, it
appears there are still many GDB installations that are linked against
Python 2.7, so we may want to keep this tiny amount of compat code
around for a while longer.

Without this commit the tests fails with errors such as:

GDB printed:
   u"std::tuple containingTypeError: iter() returned non-iterator of type '_Children'\n"
Value should match:
   u'std::tuple containing = {[1] = 2, [2] = 3, [3] = 4}'

[ldionne]

This seems like a small workaround to prevent serious headaches from people trying to use this, so LGTM.

libcxx/utils/gdb/libcxx/printers.py

class _Children(object):
 
         def __next__(self):
             # child_iter raises StopIteration when appropriate.
             field_name = next(self.child_iter)
             child = self.val["__base_"][field_name]["__value_"]
             self.count += 1
             return ("[%d]" % self.count, child)
 
+        next = __next__  # Needed for GDB built against Python 2.7.
+
     def __init__(self, val):
         self.val = val
 
     def to_string(self):
         typename = _remove_generics(_prettify_typename(self.val.type))
         if not self.val.type.fields():
             return "empty %s" % typename
         return "%s containing" % typename
class _VectorBoolIterator(object):
             else:
                 outbit = 0
             self.offset += 1
             if self.offset >= self.bits_per_word:
                 self.item += 1
                 self.offset = 0
             return ("[%d]" % self.count, outbit)
 
+        next = __next__  # Needed for GDB built against Python 2.7.
+
     class _VectorIterator(object):
         """Class to iterate over the non-bool vector's children."""
 
         def __init__(self, begin, end):
             self.item = begin
             self.end = end
             self.count = 0
 
         def __iter__(self):
             return self
 
         def __next__(self):
             self.count += 1
             if self.item == self.end:
                 raise StopIteration
             entry = self.item.dereference()
             self.item += 1
             return ("[%d]" % self.count, entry)
 
+        next = __next__  # Needed for GDB built against Python 2.7.
+
     def __init__(self, val):
         """Set val, length, capacity, and iterator for bool and normal vectors."""
         self.val = val
         self.typename = _remove_generics(_prettify_typename(val.type))
         begin = self.val["__begin_"]
         if self.val.type.template_argument(0).code == gdb.TYPE_CODE_BOOL:
             self.typename += "<bool>"
             self.length = self.val["__size_"]