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

[Carl Peto via cfe-commits]

carlos4242 wrote:

@efriedma-quic  Cool. So it sounds like it's worth parking this for now, until Kuba's work #71986 is merged?

@rjmccall I'm not 100% sure I understand? The existing code in AVR.cpp handles the standard AVR ABI, which has a few simple rules based on GCC behaviour. Here we are effectively just passing on the handling for that to `AVRABIInfo`. The error in register thing, I just copied what ARM is doing. I figure that's OK for now. If we need to do some bugfixes later to get AVR working with errors then we'll take a look then probably. Is that reasonable?

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