[AsmPrinter] Remove unused forward declarations. NFC.

Author: RKSimon

llvm/lib/CodeGen/AsmPrinter/DIEHash.h

@@ -20,7 +20,6 @@
 namespace llvm {
 
 class AsmPrinter;
-class CompileUnit;
 
 /// An object containing the capability of hashing and adding hash
 /// attributes onto a DIE.

llvm/lib/CodeGen/AsmPrinter/DwarfUnit.h

@@ -27,7 +27,6 @@
 
 namespace llvm {
 
-class MachineLocation;
 class MachineOperand;
 class ConstantInt;
 class ConstantFP;

llvm/lib/CodeGen/AsmPrinter/WinException.h

@@ -16,12 +16,10 @@
 #include "EHStreamer.h"
 
 namespace llvm {
-class Function;
 class GlobalValue;
 class MachineFunction;
 class MCExpr;
 class MCSection;
-class Value;
 struct WinEHFuncInfo;
 
 class LLVM_LIBRARY_VISIBILITY WinException : public EHStreamer {