[clang] Fix absolute file paths with -fdebug-prefix-map

Previously if you passed an absolute path to clang, where only part of
the path to the file was remapped, it would result in the file's DIFile
being stored with a duplicate path, for example:

```
!DIFile(filename: "./ios/Sources/bar.c", directory: "./ios/Sources")
```

This change handles absolute paths, specifically in the case they are
remapped to something relative, and uses the dirname for the directory,
and basename for the filename.

This also adds a test verifying this behavior for more standard uses as
well.

Differential Revision: https://reviews.llvm.org/D111352