Keep the user data for compile units up to date since we often create...

Keep the user data for compile units up to date since we often create lldb_private::CompileUnit objects without creating the DWARFCompileUnit objects when we do DWARF in .o files.

Now we make sure to update our DWARFCompileUnit -> lldb_private::CompileUnit user data when it isn't set to ensure quick transitions between the two.

<rdar://problem/18371367>

llvm-svn: 228264