Debug mode for unordered_set.  I believe this to be fairly complete for
unordered_set, however it is not complete yet for unordered_multiset,
unordered_map or unordered_multimap.  There has been a lot of work done
for these other three containers, however that work was done just to
keep all of the tests passing.

You can try this out with -D_LIBCPP_DEBUG2.  You will have to link to a
libc++.dylib that has been compiled with src/debug.cpp.  So far, vector
(but not vector<bool>), list, and unordered_set are treated.  I hope to
get the other three unordered containers up fairly quickly now that
unordered_set is done.

The flag _LIBCPP_DEBUG2 will eventually be changed to _LIBCPP_DEBUG, but
not today.  This is my second effort at getting debug mode going for
libc++, and I'm not quite yet ready to throw all of the work under the
first attempt away.

The basic design is that all of the debug information is kept in a
central database, instead of in the containers.  This has been done as
an attempt to have debug mode and non-debug mode be ABI compatible with
each other.  There are some circumstances where if you construct a
container in an environment without debug mode and pass it into debug
mode, the checking will get confused and let you know with a readable
error message.  Passing containers the other way: from debug mode out to
a non-debugging mode container should be 100% safe (at least that is the
goal).