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

[Carl Peto via cfe-commits]

carlos4242 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.

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