[clang] [AVR] make the AVR ABI Swift compatible (PR #72298)

[John McCall via cfe-commits]

rjmccall wrote:

This is less about the implementation weeds of LLVM and more about the actual details of your calling convention — the decisions about how arguments and results are passed that are ultimately downstream of the choices made here.  Mostly, I'm encouraging you as a platform maintainer who's adding Swift CC support for your platform to take a few moments to make sure you're happy with the CC.  It's likely that everything will *work* regardless — Swift will generate both declarations and calls according to the rules of the CC you specify here, which should be enough to make things functional — but you might find yourself looking at generated code in two years and realize you've been doing something ridiculous for that entire time, and maybe it'll be too awkward to change.

https://github.com/llvm/llvm-project/pull/72298