[-Wunsafe-buffer-usage] Bug fix: Handles the assertion violations for code within macros

When macros get expanded, the source location for the expanded code received by the Fixable
gadgets is invalid. We do not want to emit fixits for macro expanded code and it currently
crashes the analysis. This patch fixes the assertion violations that were introduced for
handling code with such invalid locations.

Reviewed by: NoQ, ziqingluo-90, jkorous

Differential revision: https://reviews.llvm.org/D146450