reland: [clang] Don't append the working directory to absolute paths

This fixes a bug that happens when using -fdebug-prefix-map to remap an
absolute path to a relative path. Since the path was absolute before
remapping, it is safe to assume that concatenating the remapped working
directory would be wrong.

This was originally submitted as https://reviews.llvm.org/D113718, but
reverted because when testing with dwarf 5 enabled, the tests were too
strict.

Differential Revision: https://reviews.llvm.org/D121663