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

This reverts r374324 (git commit 62808631)

I changed the test to not rely on finding the sequence "clang, test,
CoverageMapping" in the CWD used to run the test. Instead it makes its
own internal directory hierarchy of foo/bar/baz and looks for that.

llvm-svn: 374403