Implement std::exception_ptr under libsupc++.

libsupc++ does not implement the dependent EH ABI and the
functionality it uses to implement std::exception_ptr (which it
declares as an alias of std::__exception_ptr::exception_ptr) is not
directly exported to clients. So we have little choice but to hijack
std::__exception_ptr::exception_ptr's (which fortunately has the
same layout as our std::exception_ptr) copy constructor, assignment
operator and destructor (which are part of its stable ABI), and its
rethrow_exception(std::__exception_ptr::exception_ptr) function.

Also, remove some out of date comments.

Differential Revision: http://llvm-reviews.chandlerc.com/D1826

llvm-svn: 192076