Allow __fp16 as a function arg or return type for AArch64

ACLE 2.0 allows __fp16 to be used as a function argument or return
type. This enables this for AArch64.

This also fixes an existing bug that causes clang to not allow
homogeneous floating-point aggregates with a base type of __fp16. This
is valid for AAPCS64, but not for AAPCS-VFP.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216558 91177308-0d34-0410-b5e6-96231b3b80d8