Revert "Use -fdebug-compilation-dir to form absolute paths in coverage mappings"
This reverts commit f6777964bde28c349d3e289ea37ecf5f5eeedbc4.
Because the absolute path check relies on temporary path containing
"clang", "test" and "CoverageMapping" as a subsequence, which is not
necessarily true on all systems(breaks internal integrates). Wanted to
fix it by checking for a leading "/" instead, but then noticed that it
would break windows tests, so leaving it to the author instead.