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

[Ben Shi via cfe-commits]

benshi001 wrote:

> Sure thing @benshi001 ... I'll create tests. The Swift ABI is documented here: https://github.com/apple/swift/blob/main/docs/ABI/CallingConvention.rst
> 
> I don't think this change will actually change any ABI. All my code uses the normal avr-gcc ABI you referenced.
> 
> I'll work out the details with the Apple engineers on a PR in their LLVM fork. Once we've got it in a state everyone is happy with, I'll bring it back here. That way nothing is blocked and we can take the time we need. Thank you.

It would be better to mention this ABI document link in the source code.

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