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

[David Majnemer]

  Having talked to @rsmith about this issue, I'm compelled to withdraw my concerns regarding allowing types which aren't either standard or extended integer types.

  This behavior would be consistent with clang's __is_integral intrinsic.

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