[clang] [Clang] Fix 'counted_by' for nested struct pointers (PR #110497)

[Jan Hendrik Farr via cfe-commits]

Cydox wrote:

> StructAccessBase has two possible results: it finds an lvalue that's the base, which can't be peeled apart any further, or it finds a pointer that's the base. It shouldn't be possible to confuse a pointer with a MemberExpr: a MemberExpr is an lvalue, and a pointer is, in this context, an rvalue.

To confirm:
Are you saying that StructBase can't be both a MemberExpr and a pointer?

For context: I'm not familiar with the clang/llvm internals before I looked into this issue

https://github.com/llvm/llvm-project/pull/110497