X86MCTargetDesc.h - remove unnecessary MCStreamer.h include. NFC.

We don't need all of MCStreamer.h, just FormattedStream.h. The rest can be replaced with forward declarations.
X86WinAllocaExpander.cpp had an implicit dependency on MapVector.h which I've added locally.

llvm/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.h

@@ -13,21 +13,25 @@
 #ifndef LLVM_LIB_TARGET_X86_MCTARGETDESC_X86MCTARGETDESC_H
 #define LLVM_LIB_TARGET_X86_MCTARGETDESC_X86MCTARGETDESC_H
 
-#include "llvm/MC/MCStreamer.h"
 #include "llvm/Support/DataTypes.h"
+#include "llvm/Support/FormattedStream.h"
 #include <string>
 
 namespace llvm {
 class MCAsmBackend;
 class MCCodeEmitter;
 class MCContext;
+class MCInst;
+class MCInstPrinter;
 class MCInstrInfo;
 class MCObjectTargetWriter;
 class MCObjectWriter;
 class MCRegister;
 class MCRegisterInfo;
+class MCStreamer;
 class MCSubtargetInfo;
 class MCTargetOptions;
+class MCTargetStreamer;
 class Target;
 class Triple;
 class StringRef;

llvm/lib/Target/X86/X86WinAllocaExpander.cpp

@@ -19,6 +19,7 @@
 #include "X86InstrInfo.h"
 #include "X86MachineFunctionInfo.h"
 #include "X86Subtarget.h"
+#include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/PostOrderIterator.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"