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

[John McCall via cfe-commits]

rjmccall wrote:

Right, that would be the way to test it.

I don't know much about AVR, but you should also look at some of the parameters to the lowering (e.g. how many max values it's okay to break an aggregate into) and make sure you're happy with them.

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