Fix DWO breakage in r264909

Summary:
In case of Dwo, DIERef stores a compile unit offset in the main object file, and not in the dwo.
The implementation of SymbolFileDWARFDwo::GetDIE inherited from SymbolFileDWARF tried to lookup
the compilation unit in the DWO based on the main object file offset (and failed). I change the
implementation to verify the DIERef indeed references compile unit belonging to this dwo and then
lookup the die based on the die offset alone.

Includes a couple of fixes for mismatched struct/class tags.

Reviewers: tberghammer, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D18646

llvm-svn: 265011