[ARM] Armv8.2-A FP16 vector intrinsics tests

Clang support for the Armv8.2-A FP16 vector intrinsic was committed in
rC328277, but this was never followed up, i.e. the LLVM part is missing.

I've raised PR38404, and this is the first step to address this. I.e.,
this adds tests for the Armv8.2-A FP16 vector intrinsic, and thus shows
which intrinsics already work, and which need further work.

Differential Revision: https://reviews.llvm.org/D50142

llvm-svn: 338568