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

[Ilya Biryukov via llvm-commits]

https://github.com/ilya-biryukov commented:

I will double-check the builds and send another update.
(This time actually building the offending targets, sorry for a blunder yesterday)

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