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

[Utkarsh Saxena via llvm-commits]

usx95 wrote:

Sent out an [RFC](https://discourse.llvm.org/t/rfc-clang-17-0-6-would-be-minimum-version-to-build-llvm-in-c-20/75345).

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