[libomptarget] Fix compilation with libc++

[Hahnfeld]

Summary

iterator is only guaranteed to be default-constructible, without any argument.

[grokos]

LGTM

libomptarget/src/omptarget.cpp

struct LookupResult {
   struct {
     unsigned IsContained   : 1;
     unsigned ExtendsBefore : 1;
     unsigned ExtendsAfter  : 1;
   } Flags;
 
   HostDataToTargetListTy::iterator Entry;
 
-  LookupResult() : Flags({0,0,0}), Entry(0) {}
+  LookupResult() : Flags({0,0,0}), Entry() {}
 };
 
 /// Map for shadow pointers
 struct ShadowPtrValTy {
   void *HstPtrVal;
   void *TgtPtrAddr;
   void *TgtPtrVal;
 };