[clang] [Clang] Add __builtin_get_counted_by builtin (PR #102549)

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Sirraide wrote:

Also, as ever, I forgot to mention this, but this also needs a release note and it should also go in the docs next to all the other builtins (`clang/docs/LanguageExtensions.rst`).

https://github.com/llvm/llvm-project/pull/102549