[Target] Stop including bitset (NFC)

Identified with clangd.

Author: kazutakahirata

llvm/lib/Target/ARM/Thumb1FrameLowering.cpp

@@ -40,7 +40,6 @@
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MathExtras.h"
-#include <bitset>
 #include <cassert>
 #include <iterator>
 #include <vector>

llvm/lib/Target/X86/X86LoadValueInjectionRetHardening.cpp

@@ -27,7 +27,6 @@
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/IR/Function.h"
 #include "llvm/Support/Debug.h"
-#include <bitset>
 
 using namespace llvm;