[clang] [clang][FMV] Fix crash with cpu_specific attribute. (PR #115762)

[Erich Keane via cfe-commits]

erichkeane wrote:

> > Wouldn't mind it if @Fznamznon took a look though, IIRC, she did a bunch of work with cpu_specific/cpu_dispatch.
> 
> I don't think I did. I'm not familiar with this area to provide any opinion.

Ah, hrmph... maybe it was @elizabethandrews who did a bunch of multiversioning stuff lately?

https://github.com/llvm/llvm-project/pull/115762