Add a variant of GCC-style vector types for ARM NEON.

NEON vector types need to be mangled in a special way to comply with ARM's ABI,
similar to some of the AltiVec-specific vector types.  This patch is mostly
just renaming a bunch of "AltiVecSpecific" things, since they will no longer
be specific to AltiVec.  Besides that, it just adds the new "NeonVector" enum.

llvm-svn: 118724