[M68k] Fix ODR violation in GISel code

It prevents LLVM from being linked with LLD at least on Windows, with errors like this:

```
  = note: ld.lld: error: duplicate symbol: vtable for llvm::FormalArgHandler
          >>> defined at librustc_llvm-a81737dd65a7c126.rlib(M68kCallLowering.cpp.obj)
          >>> defined at librustc_llvm-a81737dd65a7c126.rlib(PPCCallLowering.cpp.obj)
```

Binutils linker also complains about this, but only with warnings.

`FormalArgHandler` has a base class `M68kIncomingValueHandler` which doesn't have a virtual method `markPhysRegUsed` like `IncomingValueHandler`s for all other targets including PPC, so it results in a conflict.
The simplest fix is to rename the `FormalArgHandler` structure (rather than to adding virtual methods for compatibility).

llvm/lib/Target/M68k/GISel/M68kCallLowering.cpp

@@ -25,6 +25,27 @@
 
 using namespace llvm;
 
+namespace {
+
+struct M68kFormalArgHandler : public M68kIncomingValueHandler {
+  M68kFormalArgHandler(MachineIRBuilder &MIRBuilder, MachineRegisterInfo &MRI)
+      : M68kIncomingValueHandler(MIRBuilder, MRI) {}
+};
+
+struct CallReturnHandler : public M68kIncomingValueHandler {
+  CallReturnHandler(MachineIRBuilder &MIRBuilder, MachineRegisterInfo &MRI,
+                    MachineInstrBuilder &MIB)
+      : M68kIncomingValueHandler(MIRBuilder, MRI), MIB(MIB) {}
+
+private:
+  void assignValueToReg(Register ValVReg, Register PhysReg,
+                        const CCValAssign &VA) override;
+
+  MachineInstrBuilder &MIB;
+};
+
+} // end anonymous namespace
+
 M68kCallLowering::M68kCallLowering(const M68kTargetLowering &TLI)
     : CallLowering(&TLI) {}
 
@@ -119,7 +140,7 @@ bool M68kCallLowering::lowerFormalArguments(MachineIRBuilder &MIRBuilder,
   CCAssignFn *AssignFn =
       TLI.getCCAssignFn(F.getCallingConv(), false, F.isVarArg());
   IncomingValueAssigner ArgAssigner(AssignFn);
-  FormalArgHandler ArgHandler(MIRBuilder, MRI);
+  M68kFormalArgHandler ArgHandler(MIRBuilder, MRI);
   return determineAndHandleAssignments(ArgHandler, ArgAssigner, SplitArgs,
                                        MIRBuilder, F.getCallingConv(),
                                        F.isVarArg());

llvm/lib/Target/M68k/GISel/M68kCallLowering.h

@@ -63,23 +63,6 @@ struct M68kIncomingValueHandler : public CallLowering::IncomingValueHandler {
                            MachinePointerInfo &MPO,
                            ISD::ArgFlagsTy Flags) override;
 };
-
-struct FormalArgHandler : public M68kIncomingValueHandler {
-  FormalArgHandler(MachineIRBuilder &MIRBuilder, MachineRegisterInfo &MRI)
-      : M68kIncomingValueHandler(MIRBuilder, MRI) {}
-};
-
-struct CallReturnHandler : public M68kIncomingValueHandler {
-  CallReturnHandler(MachineIRBuilder &MIRBuilder, MachineRegisterInfo &MRI,
-                    MachineInstrBuilder &MIB)
-      : M68kIncomingValueHandler(MIRBuilder, MRI), MIB(MIB) {}
-
-private:
-  void assignValueToReg(Register ValVReg, Register PhysReg,
-                        const CCValAssign &VA) override;
-
-  MachineInstrBuilder &MIB;
-};
 } // end namespace llvm
 
 #endif // LLVM_LIB_TARGET_M68K_GLSEL_M68KCALLLOWERING_H