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

[John McCall via cfe-commits]

rjmccall 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?

`SwiftABIInfo` exists so that targets can tweak more details than just whether to use `swifterror`.  For example, you can change the conditions under which we'll try to return things in registers, and if you don't do that, IIRC Swift will try to return quite a few things in registers by default.  I'm just saying that you should make sure that that matches with your backend — either your backend needs to make sure that it returns a type like `{ ptr, ptr, float, float }` in registers for `swiftcc` or you should tweak the frontend so that it knows to return it directly.

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