MFC r280864:

MFC r280864:

Pull in r233552 from upstream libc++ trunk (by Eric Fiselier):

  [libcxx] Fix PR22771 - Support access control SFINAE in the library
  version of is_convertible.

  Summary:
  Currently the conversion check does not take place in a context where
  access control SFINAE is applied. This patch changes the context of
  the test expression so that SFINAE occurs if access control does not
  permit the conversion.

  Related bug: https://llvm.org/bugs/show_bug.cgi?id=22771

  Reviewers: mclow.lists, rsmith, dim

  Reviewed By: dim

  Subscribers: dim, rodrigc, emaste, cfe-commits

  Differential Revision: http://reviews.llvm.org/D8461

This fixes building clang, and other programs using libc++, with newer
versions of gcc (specifically, gcc 4.8 and higher).

Reported by:	rodrigc

git-svn-id: svn://svn.freebsd.org/base/stable/10@281149 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f