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

[Brandt Bucher via cfe-commits]

brandtbucher wrote:

Thanks for working on this! It's really appreciated.

Two follow-up questions:
- Is AArch64 support planned soon?
- Am I correct in understanding that this missed the LLVM 18 cut, and won't until LLVM 19 in mid-August-ish? 

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