[RFC] Exposing ghccc calling convention as preserve_none to clang

So, what should the next step be if this is something we want to see in Clang 18? Open an issue?

Note that there are a couple additional architectures that support ghccc. These include AArch64 and RISCV. Not sure if there are any others. The LangRef still specifies that ghccc is x86/x86-64 only because (it seems like) everyone just didn’t bother to update the LangRef after doing the implementation for each architecture.

There are a couple additional architectures that could also probably easily have supported added for ghccc, namely the ones listed at https://github.com/ghc/ghc/tree/master/rts/include/stg/MachRegs that don’t already have an existing implementation within LLVM.