Refactor filename/directory in DICompileUnit into a DIFile

This is the next step towards making the metadata for DIScopes have a common
prefix rather than having to delegate based on their tag type.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176913 91177308-0d34-0410-b5e6-96231b3b80d8