[clang] [Clang] Introduce __builtin_meow_synthesises_from_spaceship (PR #155612)

[Nikolas Klauser via cfe-commits]

philnik777 wrote:

> This is clever: that way we know it's a pure optimization to call spaceship instead of less-than twice. IIUC you'd plan to use this from libc++ to optimize eg __find_equal even on user-defined types?

Yes, exactly.

https://github.com/llvm/llvm-project/pull/155612