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