Reapply "Adapt gcov to changes in CFE."

This reverts commit r348203 and reapplies D55085 with an additional
GCOV bugfix to make the change NFC for relative file paths in .gcno files.

Thanks to Ilya Biryukov for additional testing!

Original commit message:

    Update Diagnostic handling for changes in CFE.

    The clang frontend no longer emits the current working directory for
    DIFiles containing an absolute path in the filename: and will move the
    common prefix between current working directory and the file into the
    directory: component.

    https://reviews.llvm.org/D55085

llvm-svn: 348512