Pull in r199848 from upstream libc++ trunk:

  Const qualify __mem_fn call operator

  QOI improvement.

  Differential Revision: http://llvm-reviews.chandlerc.com/D2059

This should help with building recent versions of Mesa.  See also:
https://bugs.freedesktop.org/show_bug.cgi?id=75505

Reported by:	dumbbell
MFC after:	3 days