[-Wunsafe-buffer-usage] Filter out conflicting fix-its

Two fix-its conflict if they have overlapping source ranges. We shall
not emit conflicting fix-its.  This patch checks conflicts in fix-its
generated for one variable (including variable declaration fix-its and
variable usage fix-its). If there is any, we do NOT emit any fix-it
for that variable.

Reviewed by: NoQ

Differential revision: https://reviews.llvm.org/D141338