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

[Jonathan Thackray via cfe-commits]

https://github.com/jthackray approved this pull request.

C++ code looks good. Does this also fix the "+nossbs" issue we saw earlier this week?

Presumably this should be cherry-picked to the llvm19 branch?

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