[driver] Implement the -fdebug-compilation-dir in a way that is compatible with
gcc.  No test case included as I'm having problems finding a test case where
the inode/dev don't match.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180628 91177308-0d34-0410-b5e6-96231b3b80d8