<xutility>: Teach _Ptr_*_cat about contiguous iterators

[miscco]

Similar to #819 _Ptr*_cat currently only looks at plain pointers. We should expand that to contiguous_iterators

[miscco]

@CaseyCarter it is weekend, do some fun stuff

[miscco]

Oh this one led me to a beautiful ICE:

template<class T> 
inline constexpr bool bark = false;

template<class T, bool = bark<T>> 
inline constexpr bool meow = true;

template<class T> 
inline constexpr bool meow<T*> = meow<T>; 

[StephanTLavavej]

Did you get a chance to report that ICE on Developer Community for the compiler team to investigate?

[miscco]

Yes I already did and it is already marked as triaged and under consideration

[AdamBucior]

This has already been fixed by #1772.