[MC] Make MCCodeEmitter::encodeInstruction pure virtual

An empty implementation is almost always inappropriate
(DXILMCCodeEmitter::encodeInstruction is an exception).
Make the function pure virtual as before https://reviews.llvm.org/D145791

llvm/include/llvm/MC/MCCodeEmitter.h

@@ -39,7 +39,7 @@ class MCCodeEmitter {
   /// Encode the given \p Inst to bytes and append to \p CB.
   virtual void encodeInstruction(const MCInst &Inst, SmallVectorImpl<char> &CB,
                                  SmallVectorImpl<MCFixup> &Fixups,
-                                 const MCSubtargetInfo &STI) const {}
+                                 const MCSubtargetInfo &STI) const = 0;
 };
 
 } // end namespace llvm