Add nonnull; use it for atomics

Summary:
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). We can therefore use the nonnull attribute to denote that:

  - A warning should be generated if the argument is null
  - It is undefined behavior if the argument is null (because a dereference will segfault)

This patch adds support for this attribute for clang and GCC, and sticks to the subset of the syntax both supports. In particular, work around this GCC oddity:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60625

The attributes are documented:

  - https://gcc.gnu.org/onlinedocs/gcc-4.0.0/gcc/Function-Attributes.html
  - https://clang.llvm.org/docs/AttributeReference.html#nullability-attributes

I'm authoring a companion clang patch for the __c11_* and __atomic_* builtins, which currently only warn on a subset of the pointer parameters.

In all cases the check needs to be explicit and not use the empty nonnull list, because some of the overloads are for atomic<T*> and the values themselves are allowed to be null.

<rdar://problem/18473124>

Reviewers: arphaman, EricWF

Subscribers: aheejin, christof, cfe-commits

Differential Revision: https://reviews.llvm.org/D47225

llvm-svn: 333325