DebugInfo: Wrap section data and relocs together for dwarf dumping support

This is a small step that may enable some simplifications in producer
(DWARFContext) and consumer (DWARFCompileUnit and other places) by
making a more complete abstraction around the data and relocations for a
section. Small initial steps could include simple changes such as
passing the pair to DWARFCompileUnit's ctor rather than passing the data
and relocs separately. I don't intend to pursue any such changes
immediately, however.

The motivation for doing this now is that type unit dumping will need to
deal with these data+reloc pairs moreso than the existing dumping
support has needed to associate the data as type unit sections are named
the same (debug_types) and comdat group folded. So to implement dumping
and reloc handling we'll need a mapping of section->data+relocs.

llvm-svn: 191209