Add basic fission support to SymbolFileDWARF

* Create new dwo symbol file class
* Add handling for .dwo sections
* Change indexes in SymbolFileDWARF to store compile unit offset next to
  DIE offset
* Propagate queries from dwarf compile unit to the dwo compile unit
  where applicable

Differential revision: http://reviews.llvm.org/D12291

llvm-svn: 247132