[NFC][Object] Include header `BitcodeReader.h` instead of using forward declaration for BitcodeModule

[tianshilei1992]

Summary

BitcodeModule is used as element of a vector in IRSymtabFile, while in the
header there is only a forward declaration. It will work if the header BitcodeReader.h
is included before including IRObjectFile.h. However, it is not always the case,
causing compilation error. This patch simply includes the header and remove the
forward declaration.

[tejohnson]

lgtm

llvm/include/llvm/Object/IRObjectFile.h

 //===- IRObjectFile.h - LLVM IR object file implementation ------*- C++ -*-===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 //
 // This file declares the IRObjectFile template class.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_OBJECT_IROBJECTFILE_H
 #define LLVM_OBJECT_IROBJECTFILE_H
 
+#include "llvm/Bitcode/BitcodeReader.h"
 #include "llvm/Object/IRSymtab.h"
 #include "llvm/Object/ModuleSymbolTable.h"
 #include "llvm/Object/SymbolicFile.h"
 
 namespace llvm {
-class BitcodeModule;
 class Module;
 
 namespace object {
 class ObjectFile;
 
 class IRObjectFile : public SymbolicFile {
   std::vector<std::unique_ptr<Module>> Mods;
   ModuleSymbolTable SymTab;
struct IRSymtabFile {
   irsymtab::Reader TheReader;
 };
 
 /// Reads a bitcode file, creating its irsymtab if necessary.
 Expected<IRSymtabFile> readIRSymtab(MemoryBufferRef MBRef);
 
 }
 
-}
+} // namespace llvm
 
 #endif