[clang-apply-replacements] Correctly handle relative paths

The fixes from the YAML file can refer to relative paths.
Those relative paths are meant to be resolved relative to the
corresponding `build directory`.
However, `clang-apply-replacements` currently interprets all
paths relative to its own working directory. This causes issues,
e.g., when `clang-apply-replacements` is run from outside of
the original build directory.

This commit adjusts `clang-apply-replacements` to take the build
directory into account when resolving relative file paths.

Reviewed By: ymandel

Differential Revision: https://reviews.llvm.org/D112647