[libcxx-commits] [libcxx] [libc++][unique_ptr] Implement LWG 4144: Disallow unique_ptr<T&, D> (PR #209018)

[Hristo Hristov via libcxx-commits]

Zingam wrote:

> Implement [LWG 4144](https://cplusplus.github.io/LWG/issue4144) by explicitly rejecting `unique_ptr<T, D>` when `T*` is not a valid type.
> 
> This adds a pointability check and tests reference and abominable function types. It also includes a minor formatting cleanup for `unique_ptr`'s `nullptr` comparison operator.
> 
> Fixes #118359

Formatting updates are considered unrelated changes usually. If necessary they need to be submitted as NFC separately.
Please revert those changes.

https://github.com/llvm/llvm-project/pull/209018