[CodeGen] Include MachineInstr.h

This patch teaches DbgEntityHistoryCalculator.h to include
MachineInstr.h so that the header file is self contained.

Without this patch, a C++ source file that only includes
DbgEntityHistoryCalculator.h does not compile because:

  PointerIntPair<const MachineInstr *, 1, EntryKind> Instr;

wants to know the alignment of llvm::MachineInstr:

  llvm/include/llvm/Support/PointerLikeTypeTraits.h:61:28: error:
  invalid application of 'alignof' to an incomplete type
  'llvm::MachineInstr'
          detail::ConstantLog2<alignof(T)>::value;
                               ^~~~~~~~~~