DwarfUnit: Refactor DW_AT_file creation into a common function.

This is preliminary work to fix type unit file strings so they appear in
their originating CU's line table - but it's also just good/simple
cleanup, so I'm committing it ahead of time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201195 91177308-0d34-0410-b5e6-96231b3b80d8