DwarfUnit: Cleanup comments

Update comment style in `DwarfUnit`.

  - Drop duplicated comments at definition, and update the comments at
    the declaration where the definition comments looked newer or more
    complete.
  - Drop the `functionName -` prefix.
  - Add `\brief` in a few places.
  - Remove a few comments entirely that weren't adding value (just
    turned the function name and arguments into a sentence).

llvm-svn: 235345