clang and aarch64 backend changes to support Neon

[Tim Northover]

Hi Ana,

Thanks for the patches.

> Here are the initial clang and aarch64 backend changes to support Neon.

Just a note for others that I believe these are the changes that will
bring the AArch64 backend as committed in line with what has already
been reviewed so far. I don't think they need separate technical
review (though obviously if anyone *does* have comments...).

> If everyone is ok with these changes, can you Tim please go ahead and commit
> them?

As soon as I'm allowed, I will.

Tim.