[PATCH] [libc++] Teach is_integral, is_[un]signed and make_[un]signed about __[u]int128_t

[Marshall Clow]

  This all looks fine to me, EXCEPT for the types as macros.
  Let's give those types real names.

  Lets either just use __int128_t, or bikeshed a name and have a typedef.

http://llvm-reviews.chandlerc.com/D2917