Add API to extract type_info from an `std::exception_ptr`

[kammce]

We cannot use type_info because GCC and Clang prevent you from doing so when -fno-rtti is set. So instead, we can provide our API for extracting the type_info information from the exception_ptr itself. Luckily access to std::type_info is available with -fno-rtti is set.

[DolphinGui]

To be honest I'm not even sure typeinfo() is even usable on std::exception_ptr given std::exception_ptr is an opaque type. This would probably be an early implementation of P3178R1.

[kammce]

We can tack that information behind the object. It'll be apart of the exception header. Then from there we can pluck the information out. And thanks for the proposal link. That looks like a good place and API for what we want.