__c11_atomic_load's _Atomic can be 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).

clang’s lib/Headers/stdatomic.h implements these as #define to the __c11_* equivalent, which are builtins with custom typecheck. Fix the typecheck.

D47613 takes care of the libc++ side.

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

<rdar://problem/27426936>

Reviewers: rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D47618

llvm-svn: 338743