[llvm][iwyu] explicitly includes <functional> and <utility>

[cjdb]

Summary

Compiling LLVM with Clang modules and libc++ identified that
Support/Printable.h and ADL/SmallVector.h were using features that
live in these headers.

llvm/include/llvm/ADT/SmallVector.h

 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MemAlloc.h"
 #include "llvm/Support/type_traits.h"
 #include <algorithm>
 #include <cassert>
 #include <cstddef>
 #include <cstdlib>
 #include <cstring>
+#include <functional>
 #include <initializer_list>
 #include <iterator>
 #include <limits>
 #include <memory>
 #include <new>
 #include <type_traits>
 #include <utility>
 

llvm/include/llvm/Support/Printable.h

 //===--- Printable.h - Print function helpers -------------------*- 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 defines the Printable struct.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_SUPPORT_PRINTABLE_H
 #define LLVM_SUPPORT_PRINTABLE_H
 
 #include <functional>
+#include <utility>
 
 namespace llvm {
 
 class raw_ostream;
 
 /// Simple wrapper around std::function<void(raw_ostream&)>.
 /// This class is useful to construct print helpers for raw_ostream.
 ///