[clang] [llvm] [AArch64] Add a check for invalid default features (PR #104435)

[Tomas Matheson via cfe-commits]

tmatheson-arm wrote:

> > Cortex-A710 does not appear to have SSBS
> I believe this says it should be present: 

You're right, I'll fix that.

I will also actually remove FeatureCCIDX from the 8.3 mandatory features.

https://github.com/llvm/llvm-project/pull/104435