[clang] [llvm] New calling convention preserve_none (PR #76868)

[Keno Fischer via llvm-commits]

Keno wrote:

I was wondering what the motivation was to still preserve `rbp`? I guess `-fomit-frame-pointers` has fallen a bit out of favor, but I'd have expected `preserve_none`, to preserve, well, none.

https://github.com/llvm/llvm-project/pull/76868