[Debuginfo][NFC] Rename error handling functions using the same pattern.

Summary:
That patch is extracted from https://reviews.llvm.org/D74308.
Currently there are two patterns to name error handling functions:
using "Callback" and "Handler". This patch uses "Handler" for all
usage places.

Reviewers: jhenderson, dblaikie, probinson, aprantl

Reviewed By: jhenderson, dblaikie

Subscribers: hiraditya, llvm-commits

Tags: #llvm, #debug-info

Differential Revision: https://reviews.llvm.org/D74354