[BOLT] Added fuzzer target (llvm-bolt-fuzzer)

This adds a target that would consume random binary as an
input ELF file.
TBD: add structured input support (ELF).

Build:
```
cmake /path/to/llvm-project/llvm -GNinja \
-DLLVM_TARGETS_TO_BUILD="X86;AArch64" \
-DCMAKE_BUILD_TYPE=Release \
-DLLVM_ENABLE_ASSERTIONS=1 \
-DCMAKE_C_COMPILER=<sanitizer-capable clang> \
-DCMAKE_CXX_COMPILER=<sanitizer-capable clang++> \
-DLLVM_ENABLE_PROJECTS="bolt"  \
-DLLVM_USE_SANITIZER=Address \
-DLLVM_USE_SANITIZE_COVERAGE=On
ninja llvm-bolt-fuzzer
```

Test Plan: ninja llvm-bolt-fuzzer

Reviewed By: maksfb

Differential Revision: https://reviews.llvm.org/D120016

bolt/lib/Rewrite/RewriteInstance.cpp

@@ -138,7 +138,7 @@ KeepTmp("keep-tmp",
   cl::Hidden,
   cl::cat(BoltCategory));
 
-static cl::opt<bool>
+cl::opt<bool>
 Lite("lite",
   cl::desc("skip processing of cold functions"),
   cl::init(false),

bolt/tools/CMakeLists.txt

@@ -1,3 +1,4 @@
 add_subdirectory(driver)
+add_subdirectory(llvm-bolt-fuzzer)
 add_subdirectory(merge-fdata)
 add_subdirectory(heatmap)

bolt/tools/llvm-bolt-fuzzer/CMakeLists.txt

@@ -0,0 +1,7 @@
+set(LLVM_LINK_COMPONENTS
+  BOLTRewrite
+  )
+
+add_llvm_fuzzer(llvm-bolt-fuzzer
+  llvm-bolt-fuzzer.cpp
+  )

bolt/tools/llvm-bolt-fuzzer/llvm-bolt-fuzzer.cpp

@@ -0,0 +1,70 @@
+//===- llvm-bolt-fuzzer.cpp - Fuzzing target for llvm-bolt ----------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "bolt/Rewrite/RewriteInstance.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/TargetSelect.h"
+
+using namespace llvm;
+using namespace object;
+using namespace bolt;
+
+namespace opts {
+extern cl::opt<std::string> OutputFilename;
+extern cl::opt<bool> Lite;
+} // namespace opts
+
+extern "C" int LLVMFuzzerTestOneInput(const char *Data, size_t Size) {
+  const char *argv[] = {"llvm-bolt", nullptr};
+  const char argc = 1;
+  opts::OutputFilename = "/dev/null";
+  opts::Lite = false;
+
+  // Input has to be an ELF - we don't want to fuzz createBinary interface.
+  if (Size < 4 || strncmp("\177ELF", Data, 4) != 0)
+    return 0;
+  // Construct an ELF binary from fuzzer input.
+  std::unique_ptr<MemoryBuffer> Buffer =
+      MemoryBuffer::getMemBuffer(StringRef(Data, Size), "", false);
+  Expected<std::unique_ptr<Binary>> BinaryOrErr =
+      createBinary(Buffer->getMemBufferRef());
+  // Check that the input is a valid binary.
+  if (Error E = BinaryOrErr.takeError()) {
+    consumeError(std::move(E));
+    return 0;
+  }
+  Binary &Binary = *BinaryOrErr.get();
+  // Check that the binary is an ELF64LE object file.
+  auto *E = dyn_cast<ELF64LEObjectFile>(&Binary);
+  if (!E)
+    return 0;
+
+  // Fuzz RewriteInstance.
+  auto RIOrErr =
+      RewriteInstance::createRewriteInstance(E, argc, argv, "llvm-bolt");
+  if (Error E = RIOrErr.takeError()) {
+    consumeError(std::move(E));
+    return 0;
+  }
+  RewriteInstance &RI = *RIOrErr.get();
+  RI.run();
+  return 0;
+}
+
+extern "C" LLVM_ATTRIBUTE_USED int LLVMFuzzerInitialize(int *argc,
+                                                        char ***argv) {
+  llvm::InitializeAllTargetInfos();
+  llvm::InitializeAllTargetMCs();
+  llvm::InitializeAllAsmParsers();
+  llvm::InitializeAllDisassemblers();
+
+  llvm::InitializeAllTargets();
+  llvm::InitializeAllAsmPrinters();
+
+  return 0;
+}