Fix a bunch of integer template argument problems in template type diffing.

Reverts most of the changes from r168005.  Since template arguments have proper
conversions now, no extending of integers is needed.  Further, since the
integers are the correct size now, use APSInt::operator== instead of
APSInt::hasSameValue since operator== will check the size and signness match.

Prior to one comparison of APSInt's, check that both are valid.  Previous, one
could be uninitialized.  Also changed APInt to APSInt in GetInt.  This
occassionally produced a sign flip, which will now be caught by operator==.