[llvm-mca] Cleanup unnecessary includes from headers

[RKSimon]

Summary

This removes some includes/forward-declarations that don't seem to be necessary in the MCA core headers

Based off a cppclean report

[andreadb]

LGTM

llvm/include/llvm/MCA/CodeEmitter.h

 
 #ifndef LLVM_MCA_CODEEMITTER_H
 #define LLVM_MCA_CODEEMITTER_H
 
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/MC/MCAsmBackend.h"
 #include "llvm/MC/MCCodeEmitter.h"
-#include "llvm/MC/MCFixup.h"
 #include "llvm/MC/MCInst.h"
 #include "llvm/MC/MCSubtargetInfo.h"
-#include "llvm/MCA/Instruction.h"
-#include "llvm/MCA/Support.h"
 #include "llvm/Support/raw_ostream.h"
 
 #include <string>
 
 namespace llvm {
 namespace mca {
 
 /// A utility class used to compute instruction encodings for a code region.

llvm/include/llvm/MCA/HardwareUnits/LSUnit.h

 #include "llvm/ADT/SmallVector.h"
 #include "llvm/MC/MCSchedule.h"
 #include "llvm/MCA/HardwareUnits/HardwareUnit.h"
 #include "llvm/MCA/Instruction.h"
 
 namespace llvm {
 namespace mca {
 
-class Scheduler;
-
 /// A node of a memory dependency graph. A MemoryGroup describes a set of
 /// instructions with same memory dependencies.
 ///
 /// By construction, instructions of a MemoryGroup don't depend on each other.
 /// At dispatch stage, instructions are mapped by the LSUnit to MemoryGroups.
 /// A Memory group identifier is then stored as a "token" in field
 /// Instruction::LSUTokenID of each dispatched instructions. That token is used
 /// internally by the LSUnit to track memory dependencies.

llvm/include/llvm/MCA/HardwareUnits/RegisterFile.h

 #define LLVM_MCA_REGISTER_FILE_H
 
 #include "llvm/ADT/APInt.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/MC/MCRegisterInfo.h"
 #include "llvm/MC/MCSchedule.h"
 #include "llvm/MC/MCSubtargetInfo.h"
 #include "llvm/MCA/HardwareUnits/HardwareUnit.h"
-#include "llvm/Support/Error.h"
 
 namespace llvm {
 namespace mca {
 
 class ReadState;
 class WriteState;
 class WriteRef;
 

llvm/include/llvm/MCA/HardwareUnits/ResourceManager.h

 /// The classes here represent processor resource units and their management
 /// strategy.  These classes are managed by the Scheduler.
 ///
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_MCA_RESOURCE_MANAGER_H
 #define LLVM_MCA_RESOURCE_MANAGER_H
 
-#include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/MC/MCSchedule.h"
 #include "llvm/MCA/Instruction.h"
 #include "llvm/MCA/Support.h"
 
 namespace llvm {
 namespace mca {

llvm/include/llvm/MCA/Pipeline.h

 /// This file implements an ordered container of stages that simulate the
 /// pipeline of a hardware backend.
 ///
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_MCA_PIPELINE_H
 #define LLVM_MCA_PIPELINE_H
 
-#include "llvm/ADT/SmallVector.h"
-#include "llvm/MCA/HardwareUnits/Scheduler.h"
 #include "llvm/MCA/Stages/Stage.h"
 #include "llvm/Support/Error.h"
 
 namespace llvm {
 namespace mca {
 
 class HWEventListener;
 

llvm/include/llvm/MCA/Stages/DispatchStage.h

 ///
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_MCA_DISPATCH_STAGE_H
 #define LLVM_MCA_DISPATCH_STAGE_H
 
 #include "llvm/MC/MCRegisterInfo.h"
 #include "llvm/MC/MCSubtargetInfo.h"
-#include "llvm/MCA/HWEventListener.h"
 #include "llvm/MCA/HardwareUnits/RegisterFile.h"
 #include "llvm/MCA/HardwareUnits/RetireControlUnit.h"
 #include "llvm/MCA/Instruction.h"
 #include "llvm/MCA/Stages/Stage.h"
 
 namespace llvm {
 namespace mca {