[clang] clang: Remove useFP16ConversionIntrinsics target option (PR #207212)

[Nikita Popov via cfe-commits]

nikic wrote:

> > > unless the value is used in an ABI context where it needs to remain as i16.
> > 
> > 
> > Where is that handled? I don't see it in the diff.
> 
> I think this is a leftover note from the first time I tried to do this. The point is those changes aren't in the diff, the ABI isn't supposed to change. wasm-fp16.c does show the ABI IR for wasm does change, but that's fine because the ultimate codegen is the same anyway

Ah, it looks like one has to opt in using `-fnative-half-arguments-and-returns` for `__fp16` being allowed as a function arg/return (unless the target opts in). So I think we don't need to worry about ABI stability in that case.

https://github.com/llvm/llvm-project/pull/207212