[clang] [CodeGen][counted_by] See past parentheses and no-op casts (PR #151266)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

LGTM!

Thought it did raise a question of whether this code should work: https://godbolt.org/z/aoqf85r1x
```
struct S {
  int *a __counted_by(a_count);
  _Atomic int a_count;
};
```
(Not necessary as part of this PR, just something I noticed when poking around with the ignoring noop casts change.)

https://github.com/llvm/llvm-project/pull/151266