[PATCH] Allow __fp16 as a function arg or return type for AArch64

[Tim Northover]

> Are you also going to review the dependent patch, D4456?

I am thinking hard about it, but that's an ugly hack to put into Clang.

It would be better if we could use "half" unconditionally and fptrunc/fpext instead of the conversion intrinsics, but that brings its own problems (like InstCombine folding things down to real, non-trivial operations on "half").

I'm still not quite sure what the best solution is or how much work would be involved in the different approaches.

Cheers.

Tim.

http://reviews.llvm.org/D4455