Make atomic non-member functions as nonnull

Summary:
As a companion to libc++ patch https://reviews.llvm.org/D47225, mark builtin atomic non-member functions which accept pointers as nonnull.

The atomic non-member functions accept pointers to std::atomic / std::atomic_flag as well as to the non-atomic value. These are all dereferenced unconditionally when lowered, and therefore will fault if null. It's a tiny gotcha for new users, especially when they pass in NULL as expected value (instead of passing a pointer to a NULL value).

<rdar://problem/18473124>

Reviewers: arphaman

Subscribers: aheejin, cfe-commits

Differential Revision: https://reviews.llvm.org/D47229

llvm-svn: 333246