Fix clang warning "has virtual functions but non-virtual destructor".

src/IceFixups.h

 //===- subzero/src/IceFixups.h - Assembler fixup kinds ----------*- C++ -*-===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 13 matching lines @@
 using FixupKind = uint32_t;
 
 /// Assembler fixups are positions in generated code/data that hold relocation
 /// information that needs to be processed before finalizing the code/data.
 class AssemblerFixup {
   AssemblerFixup &operator=(const AssemblerFixup &) = delete;
 
 public:
   AssemblerFixup() = default;
   AssemblerFixup(const AssemblerFixup &) = default;
+  virtual ~AssemblerFixup() = default;
   intptr_t position() const {
     assert(position_was_set_);
     return position_;
   }
   void set_position(intptr_t Position) {
     position_ = Position;
     position_was_set_ = true;
   }
 
   FixupKind kind() const { return kind_; }
@@ skipping 47 matching lines @@
   const std::string Message;
   const size_t NumBytes;
 };
 
 using FixupList = std::vector<AssemblerFixup>;
 using FixupRefList = std::vector<AssemblerFixup *>;
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICEFIXUPS_H