Follow-up fix for nonnull atomic non-member functions

Handling of the third parameter was only checking for *_n and not for the C11 variant, which means that cmpxchg of a 'desired' 0 value was erroneously warning. Handle C11 properly, and add extgensive tests for this as well as NULL pointers in a bunch of places.

Fixes r333246 from D47229.

llvm-svn: 333290