[clang] Fix DIFile directory root on Windows

On unix systems this logic would not separate the file and directory of
the DIFile unless they shared more components at the start than just the
root path character. The logic to do this was unix specific so it didn't
work on Windows. Now we check if the entire root_path is the same as
what you were going to set as the Dir and use the full filepath in that
case.

Differential Revision: https://reviews.llvm.org/D111579