[analyzer] Suppress nullability warning for _Nonnull locals zero-initialized by ObjC ARC.

Prevent the analyzer from warning when a _Nonnnull local variable is implicitly
zero-initialized because of Objective-C automated reference counting. This avoids false
positives in cases where a _Nonnull local variable cannot be initialized with an
initialization expression, such as:
  NSString * _Nonnull s; // no-warning
  @autoreleasepool {
    s = ...;
  }

The nullability checker will still warn when a _Nonnull local variable is explicitly
initialized with nil.

This suppression introduces the potential for false negatives if the local variable
is used before it is assigned a _Nonnull value. Based on a discussion with Anna Zaks,
Jordan Rose, and John McCall, I've added a FIXME to treat implicitly zero-initialized
_Nonnull locals as uninitialized in Sema's UninitializedValues analysis to avoid these
false negatives.

rdar://problem/23522311

llvm-svn: 256603