[-Wunsafe-buffer-usage] Create Fix-Its only if they are emitted

`-Wunsafe-buffer-usage` diagnostics shall not emit fix-its if fix-its
are globally disabled.

Commit on behalf of jkorous (Jan Korous)

Reviewed by: NoQ (Artem Dergachev)

Differential revision: https://reviews.llvm.org/D143697