[PATCH] [libc++] Const qualify __mem_fn call operator

[Peter Collingbourne]

  As far as I can tell, provided that the client does not use reserved names, __invoke is only ever called with a function argument of type pointer to member function from __mem_fn, as those are the only template argument types that __mem_fn is ever instantiated with by mem_fn,

  In each of the overloads dealing with those function argument types, the function pointer is not (nor may it be) modified.

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