[analyzer] Add ObjCLoopChecker: objects from NSArray et al are non-nil.

While collections containing nil elements can still be iterated over in an
Objective-C for-in loop, the most common Cocoa collections -- NSArray,
NSDictionary, and NSSet -- cannot contain nil elements. This checker adds
that assumption to the analyzer state.

This was the cause of some minor false positives concerning CFRelease calls
on objects in an NSArray.

llvm-svn: 158319