[clang] [Clang] Implement the 'counted_by' attribute (PR #68750)

[Bill Wendling via cfe-commits]

bwendling wrote:

This is identical to the original change (9a954c693573281407f6ee3f4eb1b16cc545033d), but with a fix for the issue @alexfh discovered (https://github.com/llvm/llvm-project/commit/9a954c693573281407f6ee3f4eb1b16cc545033d#commitcomment-129529574). The change is on lines 473-475 of clang/lib/AST/DeclBase.cpp: using `dyn_cast_if_present` instead of just `dyn_cast`. Also added the testcase to `clang/test/CodeGen/bounds-checking.c`.

https://github.com/llvm/llvm-project/pull/68750