[clang] Turn 'counted_by' into a type attribute and parse it into 'CountAttributedType' (PR #78000)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

It's generally not a good idea to use sugar to represent constructs that are semantically significant: anything that uses the canonical type will throw it away.  We try to avoid throwing away sugar in most cases, but we aren't always successful.

It's possible there are other considerations here (maybe it matters less if the attribute is only relevant inside the definition?), but I'd like an explanation of the tradeoffs.

https://github.com/llvm/llvm-project/pull/78000