[analyzer] Nullability: fix notes around synthesized ObjC property accessors.

[analyzer] Nullability: fix notes around synthesized ObjC property accessors.

Nullable-to-nonnull checks used to crash when the custom bug visitor was trying
to add its notes to autosynthesized accessors of Objective-C properties.

Now we avoid this, mostly automatically outside of checker control, by
moving the diagnostic to the parent stack frame where the accessor has been
called.

Differential revision: https://reviews.llvm.org/D32437

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@304710 91177308-0d34-0410-b5e6-96231b3b80d8