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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294369 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/MCELFObjectTargetWriter.cpp

@@ -7,10 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "llvm/ADT/STLExtras.h"
 #include "llvm/MC/MCELFObjectWriter.h"
-#include "llvm/MC/MCExpr.h"
-#include "llvm/MC/MCValue.h"
 
 using namespace llvm;