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

[Ahmed Bougacha via cfe-commits]

https://github.com/ahmedbougacha commented:

A couple small issues inline, LGTM otherwise, thanks!

>> FEAT_SSBS is not mandatory for any architecture.
>
> https://reviews.llvm.org/D54629 says it is mandatory for 8.5-a but I can't see that in the Arm ARM.

It was indeed originally mandatory (see e.g., DDI 0487E.a) but I think it isn't anymore (hence it getting removed from later cores)

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