[clang] [BoundsSafety] WIP: Make 'counted_by' work for pointer fields; late parsing for 'counted_by' on decl attr position (PR #87596)

[Yeoul Na via cfe-commits]

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rapidsna wrote:

> This should be fine, because count is declared before use—unless I'm getting type attributes confused with field attributes..

`struct size_unknown *__counted_by(count) buf` indicates that `buf` has `sizeof(struct size_unknown) * count` bytes. So the problem is here that the compiler doesn't know what's the actual byte size of `buf` because `sizeof(struct unknown_size)` is not available here.

https://github.com/llvm/llvm-project/pull/87596