[-Wunsafe-buffer-usage] Ignore the FixableGadgets that will not be fixed at an earlier stage

`FixableGadget`s are not always associated with variables that are unsafe
(warned). For example, they could be associated with variables whose
unsafe operations are suppressed or that are not used in any unsafe
operation. Such `FixableGadget`s will not be fixed. Removing these
`FixableGadget` as early as possible helps improve the performance
and stability of the analysis.

Reviewed by: NoQ (Artem Dergachev), t-rasmud (Rashmi Mudduluru)

Differential revision: https://reviews.llvm.org/D155524