[clang] [X86] Update #error to mention the right header name (PR #207507)

[Kim Gräsman via cfe-commits]

kimgr wrote:

@phoebewang Thanks! I don't have commit access, could you merge this for me?

Actually, maybe I should address the clang-format warning -- I was considering ignoring it since the line already overruns on main, and it's handy to be able to `grep -r '#error "Never use'`, but I see other files are line-wrapped anyway. Let me know what you prefer, and I'll update before merge.

https://github.com/llvm/llvm-project/pull/207507