Refactor the DIFile (2nd) parameter to DITypes to be an MDNode reference to a raw directory/file pair

This makes DIType's first non-tag parameter the same as DIFile's, allowing them
to both share the common implementation of getFilename/getDirectory in DIScope.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177467 91177308-0d34-0410-b5e6-96231b3b80d8