[libcxx-commits] [libcxx] [libc++] Reimplement `optional<T&>` as a partial specialization (PR #201983)

[A. Jiang via libcxx-commits]

frederick-vs-ja wrote:

> ~Also fixes a bug where returning an optional<T&> for the monadic functions was broken~

I removed this from the PR message (to prevent accidentally having it in the squashed commit message). This sentence can just belong to the follow-up patch.

https://github.com/llvm/llvm-project/pull/201983