[clang] [clang][test] Add `%clang_cc1_cg_arm64_neon` substitution (PR #188547)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > Do you need one of us to land the changes on your behalf?
> 
> Thank you for the offer! I actually do have commit access (I have just landed this). Sorry about the delay - lots of personal and work distractions.

No worries, I just wanted to make sure it wasn't waiting in limbo on me. :-D Thank you!

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