[ARM] Pass _Float16 as int or float

Pass and return _Float16 as if it were an int or float for ARM, but with the
top 16 bits unspecified, similarly like we already do for __fp16.

We will implement proper half-precision function argument lowering in the ARM
backend soon, but want to use this workaround in the mean time.

Differential Revision: https://reviews.llvm.org/D42318

llvm-svn: 323185