Pull in r228705 from upstream libc++ trunk (by Eric Fiselier):

Pull in r228705 from upstream libc++ trunk (by Eric Fiselier):

  [libcxx] Fix PR 22468 - std::function<void()> does not accept
  non-void-returning functions

  Summary:
  The bug can be found here: https://llvm.org/bugs/show_bug.cgi?id=22468

  `__invoke_void_return_wrapper` is needed to properly handle calling a
  function that returns a value but where the std::function return type
  is void. Without this '-Wsystem-headers' will cause
  `function::operator()(...)` to not compile.

  Reviewers: eugenis, K-ballo, mclow.lists

  Reviewed By: mclow.lists

  Subscribers: cfe-commits

  Differential Revision: https://reviews.llvm.org/D7444

This should allow newer versions of the graphics/aseprite port to
compile without modification.

Direct commit to stable/10, since stable/11 and head already have this
change.

Reported by:	yuri@rawbw.com
PR:		213773

git-svn-id: svn://svn.freebsd.org/base/stable/10@308143 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f