[clang] C++26 Annotation (PR #166287)

[via cfe-commits]

zebullax wrote:

@katzdm I suspect you may have tried the approach from https://github.com/llvm/llvm-project/pull/166287/commits/9ddf0a8b4a6190fa8ae14e6c4e58a13c9cb02e87 instead of having a `PostInstantiate` statements copied verbatim from attr.td. 
If you recall having done so and why that didn't work out, please share 🙇 

https://github.com/llvm/llvm-project/pull/166287