Mark __c11_atomic_load as const

Summary:
C++11 onwards specs the non-member functions atomic_load and atomic_load_explicit as taking the atomic<T> by const (potentially volatile) pointer. C11, in its infinite wisdom, decided to drop the const, and C17 will fix this with DR459 (the current draft forgot to fix B.16, but that’s not the normative part).

This patch fixes the libc++ version of the __c11_atomic_load builtins defined for GCC's compatibility sake.

D47618 takes care of the clang side.

Discussion: http://lists.llvm.org/pipermail/cfe-dev/2018-May/058129.html

<rdar://problem/27426936>

Reviewers: EricWF, mclow.lists

Subscribers: christof, cfe-commits

Differential Revision: https://reviews.llvm.org/D47613

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@333776 91177308-0d34-0410-b5e6-96231b3b80d8