Fix DW_AT_specification handling in DWO files

Summary:
We were trying to get a DWARFDIE from a CompileUnit belonging to a DWO file. However, this
function does not understand the die encoding used by the DWO files. Instead use GetDIE on the
SymbolFileDWARF, which is overriden in DWO to do the right thing.

Reviewers: clayborg, tberghammer

Subscribers: lldb-commits, ovyalov

Differential Revision: http://reviews.llvm.org/D19927

llvm-svn: 268615