This adds Clang support for the ARM64 backend. There are definitely
still some rough edges, so please bring up any issues you see with
this patch.

As with the LLVM commit though, we think it'll be more useful for
merging with AArch64 from within the tree.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205100 91177308-0d34-0410-b5e6-96231b3b80d8