[-Wunsafe-buffer-usage] Refactor to let local variable fix-its and parameter...

[-Wunsafe-buffer-usage] Refactor to let local variable fix-its and parameter fix-its share common code

Refactor the code for local variable fix-its so that it reuses the
code for parameter fix-its, which is in general better. For example,
cv-qualifiers are supported.

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

Differential revision: https://reviews.llvm.org/D156189