[clang] [clang][test] Add `%clang_cc1_arm64_neon` substitution (PR #188547)

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

> If the CodeGen and AArch64 maintainers are happy, I think this is reasonable. I'll leave it to them for final sign-off.

@efriedma-quic , you are listed as a maintainer for "Clang LLVM IR generation" - are you OK with this change? Thanks!

https://github.com/llvm/llvm-project/pull/188547