[analyzer] MacOSXAPIChecker: Disallow dispatch_once_t in ivars and heap.

Unlike global/static variables, calloc etc. functions that allocate ObjC
objects behave differently in terms of memory barriers, and hacks that make
dispatch_once as fast as it possibly could be start failing.

Differential Revision: https://reviews.llvm.org/D25909

llvm-svn: 285605