clang: Remove useFP16ConversionIntrinsics target option (#207212)
Follow up to #174494. Remove the remnants of the control to
use llvm.convert.to.fp16/llvm.convert.from.fp16. Prefer
directly using the IR half type.
I did the first 80% of this a long time ago, and AI finished
the last bit and handled the recent rebases and test updates.
Co-authored-by: Claude (Opus 4.8) <noreply@anthropic.com>