[Bitcode] Add fuzzer for bitcode reading

Inspired by the discussion on D118694, this adds a straightforward
fuzzer for bitcode reading. Currently it will very quickly run into
OOM, because we do unconditional vector reservations with
user-provided sizes.

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

@@ -0,0 +1,6 @@
+set(LLVM_LINK_COMPONENTS
+  BitReader
+)
+add_llvm_fuzzer(llvm-dis-fuzzer
+  llvm-dis-fuzzer.cpp
+  )

llvm/tools/llvm-dis-fuzzer/llvm-dis-fuzzer.cpp

@@ -0,0 +1,26 @@
+//===-- llvm-dis-fuzzer.cpp - Fuzzer for llvm-dis using lib/Fuzzer --------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// Fuzzer for LLVM bitcode reading.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Bitcode/BitcodeReader.h"
+#include "llvm/Support/MemoryBuffer.h"
+
+using namespace llvm;
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
+  LLVMContext Context;
+  auto Buffer = MemoryBuffer::getMemBuffer(
+      StringRef(reinterpret_cast<const char *>(Data), Size), "Fuzzer input",
+      /*RequiresNullTerminator=*/false);
+  consumeError(
+      parseBitcodeFile(Buffer->getMemBufferRef(), Context).takeError());
+  return 0;
+}