Track undefined sym in the symtab. Remove hack for missing relocs against undef.

src/IceGlobalContext.h

 //===- subzero/src/IceGlobalContext.h - Global context defs -----*- C++ -*-===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file declares aspects of the compilation that persist across
@@ skipping 157 matching lines @@
   Constant *getConstantInt1(int8_t ConstantInt1);
   Constant *getConstantInt8(int8_t ConstantInt8);
   Constant *getConstantInt16(int16_t ConstantInt16);
   Constant *getConstantInt32(int32_t ConstantInt32);
   Constant *getConstantInt64(int64_t ConstantInt64);
   Constant *getConstantFloat(float Value);
   Constant *getConstantDouble(double Value);
   // Returns a symbolic constant.
   Constant *getConstantSym(RelocOffsetT Offset, const IceString &Name,
                            bool SuppressMangling);
+  Constant *getConstantExternSym(const IceString &Name);
   // Returns an undef.
   Constant *getConstantUndef(Type Ty);
   // Returns a zero value.
   Constant *getConstantZero(Type Ty);
   // getConstantPool() returns a copy of the constant pool for
   // constants of a given type.
   ConstantList getConstantPool(Type Ty);
+  // Returns a copy of the list of external symbols.
+  ConstantList getConstantExternSyms();
 
   const ClFlags &getFlags() const { return Flags; }
 
   bool isIRGenerationDisabled() const {
     return ALLOW_DISABLE_IR_GEN ? getFlags().DisableIRGeneration : false;
   }
 
   // Allocate data of type T using the global allocator.
   template <typename T> T *allocate() { return getAllocator()->Allocate<T>(); }
 
@@ skipping 244 matching lines @@
   explicit OstreamLocker(GlobalContext *Ctx) : Ctx(Ctx) { Ctx->lockStr(); }
   ~OstreamLocker() { Ctx->unlockStr(); }
 
 private:
   GlobalContext *const Ctx;
 };
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICEGLOBALCONTEXT_H