[libc++] Fix std::make_exception_ptr interaction with ObjC (#135386)
Clang treats throwing/catching ObjC types differently from C++ types,
and omitting the `throw` in `std::make_exception_ptr` breaks ObjC
invariants about how types are thrown/caught.
Fixes #135089
Co-authored-by: Louis Dionne <ldionne.2@gmail.com>