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

[Trevor Gross via llvm-commits]

tgross35 wrote:

This is based on top of https://github.com/llvm/llvm-project/pull/152708 which should land first, the second commit here is the relevant part.

@benshi001 could you review this as well? Cc @Patryk27 

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