[llvm] [AVR] Change `half` to use `softPromoteHalfType` (PR #152783)

[Ben Shi via llvm-commits]

benshi001 wrote:

AVR is a quite simple 8-bit CPU, it lacks HW float unit and vector unit. In real production environment, clang-AVR relies on libgcc-7.3, which also lacks infrastrure of float16. In fact avr-gcc 7.3 even attributes double to float, as indicated in https://gcc.gnu.org/wiki/avr-gcc.

So I think the best way is also attributing half type to float32, to fit avr-gcc-7.3 (and its libgcc) which is the main stream version in real production environment.

However we can not prevent hand writing `double` / `half` in llvm IR, so I am not sure how to deal with that, please let me have a carefully consideration.

https://github.com/llvm/llvm-project/pull/152783