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

The last attempt at https://github.com/llvm/llvm-project/pull/72220 was
reverted by
https://github.com/llvm/llvm-project/commit/94d6699bf5eeb5aa4c50d1d90f8bf69b79201ceb
because it breaks C++20 build in clang-17 and before.

This is a workaround of
https://github.com/llvm/llvm-project/issues/70210 and unblocks
https://github.com/llvm/llvm-project/pull/72213 which rectifies
rewriting template operator and thus introduces new breakages.

Moving the function definition out of the class makes clang find a
matching `operator!=` for the `operator==`. This makes clang not rewrite
the `operator==` with reversed args. Hence, the ambiguity is resolved.

The final plan, when https://github.com/llvm/llvm-project/issues/70210
is fixed, is to move these back to inline definition or even convert to
a member template operator. This should not be urgent and could even
wait for a major clang release including
https://github.com/llvm/llvm-project/pull/72213