[clang] [BoundsSafety][Sema] Allow `sized_by`/`sized_by_or_null` on pointers to structs with a flexible array member (PR #209603)

[Dan Liew via cfe-commits]

delcypher wrote:

Merged now because this is blocking a downstream diagnostic refactoring. The end result of that will be upstreamed and unifies some of the diagnostic handling for `__counted_by` with `-fbounds-safety` on and off. Please let me know if there is additional feedback on this PR. I'm happy to address if needed.

https://github.com/llvm/llvm-project/pull/209603