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

[Caleb Helbling via cfe-commits]

calebh wrote:

We have another use case for this calling convention - micropatching. See my coworker's blog post on how this would work: https://www.philipzucker.com/permutation_compile/

Is there a reason why r10 isn't included in the list of registers for x64?

Having support for additional architectures (ARM, PPC) would be great for our use case.

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