[ORC] Add NullResolver

[JosephTremoulet]

Summary

This is an implementation of RuntimeDyld::SymbolResolver that simply
rejects all resolution requests; useful for clients that do not have any
cross-object symbol references.

[lhames]

Looks good to me. Thanks Joseph!

include/llvm/ExecutionEngine/Orc/NullResolver.h

+//===------ NullResolver.h - Reject symbol lookup requests ------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+//   Defines a RuntimeDyld::SymbolResolver subclass that rejects all symbol
+// resolution requests, for clients that have no cross-object fixups.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_EXECUTIONENGINE_ORC_NULLRESOLVER_H
+#define LLVM_EXECUTIONENGINE_ORC_NULLRESOLVER_H
+
+#include "llvm/ExecutionEngine/RuntimeDyld.h"
+
+namespace llvm {
+namespace orc {
+
+/// SymbolResolver impliementation that rejects all resolution requests.
+/// Useful for clients that have no cross-object fixups.
+class NullResolver : public RuntimeDyld::SymbolResolver {
+public:
+  RuntimeDyld::SymbolInfo findSymbol(const std::string &Name) final;
+
+  RuntimeDyld::SymbolInfo
+  findSymbolInLogicalDylib(const std::string &Name) final;
+};
+
+} // End namespace orc.
+} // End namespace llvm.
+
+#endif // LLVM_EXECUTIONENGINE_ORC_NULLRESOLVER_H

lib/ExecutionEngine/Orc/CMakeLists.txt

 add_llvm_library(LLVMOrcJIT
   ExecutionUtils.cpp
   IndirectionUtils.cpp
+  NullResolver.cpp
   OrcMCJITReplacement.cpp
   OrcTargetSupport.cpp
 
   ADDITIONAL_HEADER_DIRS
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/ExecutionEngine/Orc
   )

lib/ExecutionEngine/Orc/NullResolver.cpp

+//===---------- NullResolver.cpp - Reject symbol lookup requests ----------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ExecutionEngine/Orc/NullResolver.h"
+
+#include "llvm/Support/ErrorHandling.h"
+
+namespace llvm {
+namespace orc {
+
+RuntimeDyld::SymbolInfo NullResolver::findSymbol(const std::string &Name) {
+  llvm_unreachable("Unexpected cross-object symbol reference");
+}
+
+RuntimeDyld::SymbolInfo
+NullResolver::findSymbolInLogicalDylib(const std::string &Name) {
+  llvm_unreachable("Unexpected cross-object symbol reference");
+}
+
+} // End namespace orc.
+} // End namespace llvm.