Dwarf parser: don't lookup void typedefs in the DWO

Summary:
void typedefs do not have a DW_AT_type attribute, so we end up with an empty encoding_uid
variable. These don't need to be looked up and trying to look that will assert in a debug build.

Reviewers: clayborg

Subscribers: lldb-commits, tberghammer

Differential Revision: http://reviews.llvm.org/D22218

llvm-svn: 275164