[libcxx-commits] [libcxx] [libcxx] changes `__is_allocator` from a struct to a variable or concept (PR #68629)

[via libcxx-commits]

https://github.com/philnik777 requested changes to this pull request.

In general I really like this, but we'll have to wait a bit until we can land this due to libc++ policy.

https://github.com/llvm/llvm-project/pull/68629