[llvm] [STLExtras] Add out-of-line definition of friend operator== for C++20 (PR #72348)

[Utkarsh Saxena via llvm-commits]

usx95 wrote:

This would need a cherry-pick for issue https://github.com/llvm/llvm-project/issues/68901 to get into 17.0.6. After that we should be able to submit this and upgrade C++20 bot to 17.0.6.

https://github.com/llvm/llvm-project/pull/72348