Tidy up some things in <arm_neon.h>.

Tidy up some things in <arm_neon.h>.
Stop defining types with "__neon_" prefixes and then using typedefs without
the prefix; there's no reason to do that anymore.  Remove types that combine
multiple Neon vectors and treat them as a single long vector; they are not
used.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@119369 91177308-0d34-0410-b5e6-96231b3b80d8