DebugInfo: Just store the DIE by value in the DwarfUnit

Since all 4 ctor calls in DwarfDebug just pass in a trivially
constructed DIE with the right tag type, sink the tag selection down
into the Dwarf*Unit ctors (removing the argument entirely from callers
in DwarfDebug) and initialize the DIE member in DwarfUnit.

llvm-svn: 207448