[clang] [Clang] Fix potential null pointer dereferences in retain cycle detection (PR #95192)

[John McCall via cfe-commits]

rjmccall wrote:

Okay.  It looks like it's actually impossible for this to be null — since we're looking at a `super` dispatch, we must be inside an Objective-C method declaration.  We do appreciate people running static analysis on our code base, but please think of analysis reports as the start of an investigation rather than something that needs to be reflexively fixed.

https://github.com/llvm/llvm-project/pull/95192