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

[labath]

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.

[clayborg]

Looks good.

lldb/trunk/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp

if (die)
                                     default:
                                     case DW_AT_sibling:
                                         break;
                                 }
                             }
                         }
                     }
 
-                    if (tag == DW_TAG_typedef)
+                    if (tag == DW_TAG_typedef && encoding_uid.IsValid())
                     {
                         // Try to parse a typedef from the DWO file first as modules
                         // can contain typedef'ed structures that have no names like:
                         //
                         //  typedef struct { int a; } Foo;
                         //
                         // In this case we will have a structure with no name and a
                         // typedef named "Foo" that points to this unnamed structure.