Install: add arm_neon.h header back

I'd gone too far pruning aarch64_simd.h this time and took out one
instance of arm_neon.h. This should restore us to the status quo.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205111 91177308-0d34-0410-b5e6-96231b3b80d8