exception_ptr.h needs the forward declaration because it's
included from <typeinfo> before <typeinfo> defines std::type_info.
Tested:
bootstrap + make check-c++ on x86_64.
libstdc++-v3/ChangeLog:
2011-03-31 Jeffrey Yasskin <jyasskin@google.com>
* libsupc++/exception_ptr.h: Forward-declare std::type_info.
* libsupc++/nested_exception.h (__throw_with_nested): Remove a
redundant default argument from std::__throw_with_nested.
From-SVN: r171807
