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

[Ben Shi via cfe-commits]

benshi001 wrote:

I am not familiar with swift. So is there an official document for the SWIFT specific ABI on AVR? I think this is necessary, just like C++: https://gcc.gnu.org/wiki/avr-gcc.

And what's more, tests are required to be committed with functional changes, this is a requirement for all llvm commits with few exception.

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