[MC] Fix some Clang-tidy modernize and Include What You Use warnings; other minor fixes (NFC).

llvm-svn: 294526
diff --git a/llvm/lib/MC/MCAsmBackend.cpp b/llvm/lib/MC/MCAsmBackend.cpp
index 570f764..fc0aa78 100644
--- a/llvm/lib/MC/MCAsmBackend.cpp
+++ b/llvm/lib/MC/MCAsmBackend.cpp
@@ -1,4 +1,4 @@
-//===-- MCAsmBackend.cpp - Target MC Assembly Backend ----------------------==//
+//===- MCAsmBackend.cpp - Target MC Assembly Backend ----------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
@@ -7,14 +7,19 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "llvm/MC/MCAsmBackend.h"
+#include "llvm/ADT/None.h"
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/MC/MCAsmBackend.h"
 #include "llvm/MC/MCFixupKindInfo.h"
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+
 using namespace llvm;
 
-MCAsmBackend::MCAsmBackend() {}
+MCAsmBackend::MCAsmBackend() = default;
 
-MCAsmBackend::~MCAsmBackend() {}
+MCAsmBackend::~MCAsmBackend() = default;
 
 Optional<MCFixupKind> MCAsmBackend::getFixupKind(StringRef Name) const {
   return None;