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

[Phoebe Wang via cfe-commits]

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

Looks reasonable given https://github.com/llvm/llvm-project/commit/a2d309912a2863dfe7286ffde67b968e8c720b07 is for ifunc only.

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