`<exception>`: Optimize `exception_ptr` a bit

[frederick-vs-ja]

It seems that we can optimize exception_ptr a bit without breaking ABI, given the internal implementation details actually use shared_ptr.

The following is possibly the full list but I'm not very sure.

• the default constructor and constructor from nullptr_t can just rely on the default member initializers (initializing both internal pointer members to nullptr).
• the swap function can just std::swap both pointer members.
• the operator== can just compare _Data1 (which is equal to the result of get() of the internally reinterpreted shared).
• the assignment operator from nullptr_t can swap *this with the empty exception_ptr first, which avoids one uninlined call to __ExceptionPtrAssign.
• operator bool() can just return _Data1 != nullptr.

[StephanTLavavej]

I have no objections to such optimizations; there's no need to call the separately compiled functions when we know the representation.