[Sema] Warn unused functions for FMV based on the target attribute (#81302)

The spurious -Wunused-function warning issue for `target_version` #80227
also applied to `__attribute__((target(...)))` based FMV. #81167 removed
warnings for all `target`-based FMV. This patch restores the warnings
for `__attribute__((target("default")))`.