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

src/IceGlobalContext.h

Index: src/IceGlobalContext.h
diff --git a/src/IceGlobalContext.h b/src/IceGlobalContext.h
index 04f08a2dbffa3191633fb39343b9e6e03f1ef1e0..3a7f755f3e4f57055f92ff27836a947adc10464b 100644
--- a/src/IceGlobalContext.h
+++ b/src/IceGlobalContext.h
@@ -175,6 +175,7 @@ public:
   // 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.
@@ -182,6 +183,8 @@ public:
   // 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; }