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

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cor3ntin wrote:

@usx95 stupid question, how does that affects llvm when built with GCC/MSVC?
If we do require a too recent version of compiler to build in C++20 mode it will make it harder to move llvm to C++20.


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