[clang] [Bounds-Safety] Add sized_by, counted_by_or_null & sized_by_or_null (PR #93231)

[Henrik G. Olsson via cfe-commits]

hnrklssn wrote:

@delcypher @rapidsna updated __sized_by(_or_null) to only allow types with unknown size if incomplete

https://github.com/llvm/llvm-project/pull/93231