Major DWARF cleanup.

Added a new class called DWARFDIE that contains a DWARFCompileUnit and DWARFDebugInfoEntry so that these items always stay together.

There were many places where we just handed out DWARFDebugInfoEntry pointers and then use them with a compile unit that may or may not be the correct one. Clients outside of DWARFCompileUnit and DWARFDebugInfoEntry should all be dealing with DWARFDIE instances instead of playing with DWARFCompileUnit/DWARFDebugInfoEntry pairs manually.

This paves to the way for some modifications that are coming for DWO.

llvm-svn: 246100