[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/MCDisassembler/MCSymbolizer.cpp b/llvm/lib/MC/MCDisassembler/MCSymbolizer.cpp
index c0f707d..78e611e 100644
--- a/llvm/lib/MC/MCDisassembler/MCSymbolizer.cpp
+++ b/llvm/lib/MC/MCDisassembler/MCSymbolizer.cpp
@@ -1,4 +1,4 @@
-//===-- llvm/MC/MCSymbolizer.cpp - MCSymbolizer class -----------*- C++ -*-===//
+//===-- llvm/MC/MCSymbolizer.cpp - MCSymbolizer class ---------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
@@ -11,5 +11,4 @@
 
 using namespace llvm;
 
-MCSymbolizer::~MCSymbolizer() {
-}
+MCSymbolizer::~MCSymbolizer() = default;