Revert "Use -fdebug-compilation-dir to form absolute paths in coverage mappings"

This reverts commit f6777964.

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.

llvm-svn: 374324