Fix broken assert in DWARFDIE.cpp on OS X

[tfiala]

Summary

This addresses an assert that has started firing recently on the OS X test suite.

The change was proposed by the assert's author, Tamas Berghammer. (Thanks for the fix, Tamas!)

[tberghammer]

Looks good

[tfiala]

Closed by commit:

$ svn commit
Sending        source/Plugins/SymbolFile/DWARF/DWARFDIE.cpp
Transmitting file data .
Committed revision 250426.

source/Plugins/SymbolFile/DWARF/DWARFDIE.cpp

if (die_offset != DW_INVALID_OFFSET)
         SymbolFileDWARF *dwarf = GetDWARF();
         if (dwarf)
             id = dwarf->MakeUserID(die_offset);
         else
             id = die_offset;
 
         if (m_cu)
         {
-            lldb::user_id_t cu_id = ((lldb::user_id_t)m_cu->GetID())<<32;
+            lldb::user_id_t cu_id = m_cu->GetID()&0xffffffff00000000ull;
             assert ((id&0xffffffff00000000ull) == 0 ||
                     (cu_id&0xffffffff00000000ll) == 0 ||
                     (id&0xffffffff00000000ull) == (cu_id&0xffffffff00000000ll));
             id |= cu_id;
         }
         return id;
     }
     return LLDB_INVALID_UID;